A. Lawson, at Brancepeth, near Durham, is so perfectly similar to its appearance as drawn and des ribed to me by another observer at Woodburn, at the same hour on the same morning, about twenty-miles north-west from Newcastle, and about thirty miles from Durham. that its unusually bright appearance near Durham may not impossibly correspond with equally favourable views of it obtained by observers at more distant places. The sky, which remained clear during the day, clouded over towards midnight on the 13th, and the stars were completely hidden during the remainder of the night. A slight rain, which began in the morning, also continued to fall during the day of the 14th, and the sky here remained entirely overcast on that evening until after midnight. Shortly before four o'clock on the morning of the 15th the clouds cleared off, and the appearance of several meteors, one of which was as bright as Jupiter, gave evident signs of the progress of the November star shower. The perfect clearness and darkness of the sky, in the absence of the moon, at the same time gave e pecial brightness to the meteors and to their phosphorescent streaks Between four o'clock and the first approach of daylight, at six o'clock, thirty-two meteors were counted, or at the rate of sixteen per hour, of which three were as bright, or brighter, than first magnitude stars, nine as bright as second, six as bright as third, and eight no brighter than stars of the fourth or lesser magnitudes. Twenty-six of these meteors were directed from the usual radiant point in Leo, which on this occasion, although not very well defined, appeared to be approximately close to the star Zeta, in Leo's sickle. About one half of their number left persistent streaks, which sometimes appeared to grow brighter after the meteors had disappeared, and I vainly endeavoured to bring them into the field of view of the directvision prisms of a small spectroscope, the duration of the brightest streaks noted scarcely ever exceeding one or two secon s. A very brilliant meteor, casting around a flash like that of lightning, was seen here shortly after nine o'clock on the evening of the 13th (and its appearance was also noted at Woodburn), traversing the north-west sky. The e particulars, imperfect as they were, unfortunately, rendered by the cloudy weather, are the only descriptions of the November star-shower which its appearance here has hitherto enabled me to supply. Newcastle-on-Tync, Nov. 17

A. S. HERSCHEL

"I had occasion to be at the station at 8.30 A.M. I then first saw them. The night had been hard frost with a clear sky. The ground was covered with hoar. There was no mist. The sun was intensely bright, but the air was very chilly. I went home and looked at my thermometer in the porch at the north side of my house; it stood at 29° F. I then went to the top of a hill to have a better view. I instantly made a sketch of the phenomenon,

a copy of which I enclose. The lower art of the circle was hidden by a bank of dark clouds. The upper part presented the most marked appearance, and was intensely white. The lump to the north side was more intense in colours than the southern, but both were distinct as to quantity of reflected light. The colours were prismatic, but a bright amber prevailed. The disappearance began at a few minutes before ten, and by five minutes past ten all had cleared away. With the exception of the bank of clouds beneath, there were only a few pencils of cirrus cloud in the sky.

"Brancepeth, Durham, Nov. 13"

Paraselene

IN NATURE, Nov. 9, there appeared the description of a remarkable paraselene observed at Highfield House on the 25th of Oct. A similar phenomenon was seen at Penrith the same night from about 10.30 to II. As this, however, differed altogether in detail from that observed by Mr. Lowe, I now offer a sketch of what we saw.

Thin mists and white flying scuds travelled across the sky. A luminous ring of perhaps at a guess 150° radius encircled the Within this was a cross of the same brightness as the encircling ring. The bars of the cross were to the eye horizontal

moon.

and vertical, intersecting in the moon. Where the horizontal bar cut the luminous ring there were bright patches of light (mock moons), rivalling the moon, as seen through the mist, in brilliancy, but without its defined outline. Where the vertical bar cut the

ring there was no increase of brightness. Such a portent in ages gone by might well have filled crusaders with hope, and perhaps thus turned the tide of battle on the morrow. We may make a useful note for future guidance by remarking what followed its appearance in this district. Up to the 25th we had for some time had very fine weatherr After the 25th we had five stormy days of wind and rain. T. MCK. HUGHES

The Solar Parallax

PROF. NEWCOMB wishes apparently to make this discussion as personal as possible. Though I do not intend to follow him in this respect, I must answer him.

He asserts that my abstract of his notes was inadequate; that I"hid the point of the most remarkable of" my "inaccuracies, and ignored the imperfections entirely." This is not so. My abstract was strictly accurate and very much fuller than the utter triviality of his objections warranted. I distinctly stated why I did not discuss the matters which he is pleased to regard as imperfect-his comments being too vague. But this was not ignoring them. His memoranda were not in a state to be printed in full, nor d'd he even hint that he wished them to be.

As he himself characterises my mistake about his own researches as "the most remarkable of my inaccuracies," it is fortunate that this mistake is also one I am forced to explain at length, owing to the tone Prof. Newcomb has taken respecting it. I certainly did omit a part of Prof. Newcomb's charge; but in his own interest, for it was worded in the very tone to which I now take exception.

In the first place, it is not to be inferred that, because an author comments on uch and such a work, he thereby wishes it to be understood that he has himself studied the original memoir in which the work was presented to the world. For instance: many very eminent men have commented on the work of Adams and Leverrier in the matter of Neptune who have not read a line of the original reasoning of these astronomers. That I, of all men (who have expressed something like contempt for memoirhunting, and have always cared rather to explain methods and describe facts than to write the history of astronomy), should be expected to read every memoir to which I refer, is preposterous in the extreme. It may seem only natural to Prof. Newcomb that when I heard of his having discussed the transit of Venus, I should hurry to obtain his memoir that I might study it ab initio usque ad finem; but, as a matter of fact, a paper of the sort, even if placed in my hands, would scarcely tempt me to take up my paper-knife.

Here are the facts of the case.

I read in the Astronomical Register a letter which may be called

anonymous, if we please, but which was referred by every one who read it to the Astronomer-Royal for Scotland, who showed not the slightest wish to conceal his identity. Doubtless on hearsay evidence (in which, however, he placed, I am sure, as much reliance as I placed in his own statement), Prof. Smyth asserted that Newcomb had anticipated Stone's labours. I took

it for granted that it was so, since I saw no room or reason for doubt. There was my error. But, says Prof. Newcomb, whence comes the value 8'87 "which it will be noted is Mr. Petrie's pyramid value?" and on what does Mr. Proctor found his comments "about my treatment of contacts? I am as much in the dark as ever." I will tell him. The value 8'87 has nothing on earth to do (so far as I am concerned) with Mr. Petrie's pyramid value. It is simply the value insisted upon by Prof. Newcomb in a paper which appeared in the Monthly Notices of the Royal Astronomical Society for November 1868; respecting which Mr. Stone remarked (see the same number of the Notices) that "the point M. Newcomb has raised is a question of only 004, viz. between my value and 8" 87-a question, therefore, of comparative insignificance." Most just remark! With my belief as to Prof. Newcomb's prior work, was it wonderful that I concluded that 8" 87 was his own pet figure for the parallax? Then it chanced that the Royal Astronomical Society, venturing to ignore Prof. Newcomb's objections, bestowed on Mr. Stone, in 1869, the Gold Medal of the Society for his researches into the Venus transit; and in the remarks which accompanied the presentation, it was stated that all preceding researches were imperfect in this respect, that (to use my own words) "no fixed rule had been adopted for interpreting the observations of internal contact. Prof. Newcomb cannot fail to see how this statement accounts for the estimate (not my estimate) of his supposed researches.

As a matter of fact, however-apart from the inference to which Prof. Newcomb is so anxious to give point—I am somewhat hardly treated in this matter. When I came to the part of my book where Prof. Newcomb's supposed researches should be dealt with, I thought thus in my mind: "Assuredly Newcomb has done this thing, for Prof. Smyth says so. Shall I leave his researches unnoticed because I can find no trace of them? That would be scarcely fair. Moreover, he is an American, and to omit all notice of his work will be so much the more objectionable. Verily I will repeat the statement of my esteemed friend at Edinburgh, and I will combine with it the weighty judgment of my friends at the council-board of the Astronomical Society. Thus will the researches of Newcomb be recorded, and due credit be assigned to him for his industry and skill, while yet no undue weight will be given to the numerical result of his labours." That I thus fell into error I have already admitted. But the error is venial in its nature, and utterly insignificant in its effects. As I am conscious that it arose chiefly from my desire (shown in other ways and places) to do justice to our American fellowworkers in science, I am in no way ashamed of it; and I conceive that Prof. Newcomb should have been the last to comment in the manner he has done on the subject.

I shall not follow him in his discussion respecting irradiation, leaving Mr. Stone to deal, in his own good time, with the arguments by which two Continental astronomers (and one American mathematician) have sought to deprive him of his justly-earned credit.

I would submit, in conclusion, that February 1869 (the date of the presentation of the Astronomical Society's medal to Mr. Stone) can scarcely be described as "five years" ago even now, and my treatise on the sun was published in February 1871, Chapter I. being in type in November 1870. Nor has the council of the Astronomical Society (or any member of it) expressed any doubt, as yet, regarding the justice of the decision arrived at in 1869. Yet not a few members of the council have paid marked attention to Prof. Newcomb's attacks upon Mr. Stone. Verbum RICH. A. PROCTOR

sat.

Brighton, Nov. 24

The Density and Depth of the Solar Atmosphere THE demonstration relating to the density and depth of the solar atmosphere, published in NATURE October 5, 1871, page 449, has been entirely misconceived by Mr. Ball. The volume of the terrestrial atmosphere is an element which obviously has nothing to do with the question. Atmospheric air, if raised to a temperature of 3,272,000° Fah., will expand 6,643 times; hence a vertical column forty-two miles high will reach a height of

27.9

=

279.006 miles, if brought to the stated temperature. The basis of computation adopted by Captain Ericsson being an area of one square inch, he shows that a medium similar to the terrestrial atmosphere containing an equal quantity of matter for corresponding area, transferred to the solar surface, will, owing to the superior attraction of the sun's mass, exert a pressure of 14'7 × 279 410 pounds. And that, if the said medium be heated to a mean temperature of 3,272,000° Fah., it will expand to a height of 279'006 10,000 miles above the solar surface. But, if a gas composed chiefly of hydrogen 14 times heavier than hydrogen the specific gravity of which is of that of air, be substituted, the height will be 14 × 10,000 = 100,000 miles. Admitting that the ascertained coefficient of expansion, o'00203 for 1° Fah., holds good at the high temperature before referred to, the stated altitudes of the solar atmosphere cannot be disputed. Mr. Ball's announcement concerning the properties of spheres, it is scarcely necessary to observe, has no bearing on the foregoing calculations. With reference to the effect of intense heat, it will be well to bear in mind that the before-mentioned rate of expansion holds good for atmospheric air-within an insignificant fraction-under extreme rarefaction as well as under high temperatures. We have no valid reason, therefore, to suppose that any deviation from the ascertained law of expansion takes place in the solar atmosphere, sufficient to alter materially the before-mentioned computations of its depth.

I'4

Mr. Ball, in expressing the opinion that we shall not gain much correct knowledge of the solar atmosphere by the inquiry instituted by Captain Ericsson. forgets that the retardation which the radiant heat suffers in passing through our atmosphere has been ascertained, and that the properties of atmospheric air with reference to temperature and expansion are nearly identical with those of hydrogen, now admitted to be the chief constituent of the solar atmosphere. It is evident that Mr. Ball does not comprehend the object of adopting the terrestrial atmosphere as a means of determining the extent and depth of the solar atmosphere, since he does not perceive that the comparison instituted by Captain Ericsson has brought out the fact that either the depth of the sun's atmosphere exceeds 100,000 miles, or it contains less gaseous matter than the earth's atmosphere for equal area. The importance of this conclusion with regard to the determination of the retardation of the radiant heat in passing through the sun's atmosphere is self-evident to all who regard solar radiation as energy which cannot be absorbed unless an adequate amount of matter be present. Mr. Ball's suggestion that the retardation depends on the "chemical, i.e. molecular-constitution" of the solar atmosphere, calls to mind how libly some physicists talk of "arresting" one half, or more, of the solar energy. These reasoners apparently do not perceive that the means of arre ting such stupendous energy is more difficult to conceive than the means of producing it.

Respecting the experiments which have been made with incandescent cast-iron spheres, and inclined discs, it is important to mention that previous experiments had established the fact that the radiant heat of flames transmits equal temperature, under similar conditions, as incandescent cast iron. The inference, therefore, which has been drawn by Captain Ericsson from the results of his experiments with incandescent cast-iron spheres regarding the feebleness of radiant heat emanating from the sun's border is not unwarrantable as supposed by Mr. Ball. New York, Nov. 10

An Aberrant Foraminifer

I CHANCED upon an aberrant form of Peneroplis the other day, in which the free terminal series of chambers of this Foraminifer, ordinarily single, is constricted into two distinct tubes.

Though new to me, it may not be sɔ to some of your readers; Dr. Carpenter, however, does not mention it in his monograph.

St. John's College, Cambridge

THULE

W. JOHNSON SOLLAS

"New Original Observation "

ERNST FRIEDINGER, of Vienna, begins a communication on the subject of "which cells in the gastric glands secrete the

84

pepsine ? as follows:-"Kölliker erwähnt zuerst das Vorkommen von zweierlei Zellen in den Pepsindrüsen des Hundes." On referring to Kölliker I find, " Bei Thieren sind, wie Todd-Bowman zuerst beim Hunde, ich und Donders bei vielen andern Säugern gezeigt haben, die Magendrüsen überall doppelter Art," &c. In Todd and Bowman, published some years before this, the two kinds of glands are figured (the drawings being better than those of Kölliker), the difference between them in anatomical characters, the difference of the two parts of the gland, and the difference in the function discharged by the two kinds of cells of each of the two kinds of glands, pointed out. Friedinger does not even L. S. B. mention the names of the English observers.

New Zealand Forest-Trees

IN your paper of Nov. 9 I observed a letter about New Zealand Forest-Trees, signed by Mr. John R. Jackson of Kew. Mr. Jackson refers to several of the magnificent varieties of forest trees belonging to the natural order of Coniferæ, which are widely distributed in New Zealand; omitting, however, some of the most common and most valuable, especially the This tree affords Kahikatea or "white pine" of the settlers. timber of a white colour, much like yellow deal in appearance and quality, which is admirably adapted for use as weatherboard, flooring-boards, and scantling for all in-door work as well as for ordinary furniture. It is most extensively used for all those purposes. The "Totara " is particularly used for making shingles, which form a good substitute for slates as a covering for roofs.

The Rimu is used for such work as requires a more durable wood, and for the making of superior furniture, the wood being much harder and more difficult to work, than that of the Kahikatea, while its beautiful colour renders it very suitable for ordinary cabinet work.

Varieties of the acacia, called Kowai by the natives, supply timber which is specially adapted for the making of pales and fencing, and which is as durable as English oak; and there are many varieties of trees suitable for all purposes.

It is, however, in reference to that which is mentioned as the

"Makia" that I think it worth while to trouble you, as I believe that I may be able to suggest what the word so referred to really is. I know of no tree or shrub so called, but Manuka, pronounced Manooka, is the name of the tree from which the natives in former times used to make all sorts of implements, especially the spears, which formed at once the weapons and the sceptres of That hardly deserves to be called a forest-tree, as it the chiefs. rarely attains any great size.

It belongs, I believe, to the family of "Diosma," and its wood is used to make axe-handles, ramrods for guns, &c. The leaves have a pleasant aromatic odour, and an infusion of them forms a passable substitute for tea, to which we were frequently glad to resort in the early times of New Zealand settlements. The fresh twigs form an elastic couch, which constituted our favourite bed on exploring parties and in temporary dwellings. WILLIAM DAVISON Braintree, Nov. 20

The Food of Plants

YOUR reviewer takes exception to my empirical description of carbonic acid in "Notes on the Food of Plants," p. 23. I readily admit-and I should have thought it was unnecessary to do sothat to describe carbonic acid as "carbon dioxide combined with water" is not strictly correct; but I think it is much more likely that I should have led my unscientific readers astray, had I explained, in more accurate language, the supposed composition of this acid. CUTHBERT C. GRUNDY

The Germ Theory of Disease

IN NATURE, October 5, p. 450, Prof. Bastian, versus the "Such germs when present would be Germ Theory, says:—' sure to go on increasing until they brought about the death of their host." Now, is it not well known that the larvæ of Trichina spiralis become encysted in the muscles of the animal infested by them, and are then perfectly harmless to their host, the fever, sometimes with fatal results, being produced by the * Aus dem lxiv. Bande der Sitzb, der k. Akad. der Wissensch. II. Abth. Oct -Heft. Jahrg. 1871.

THE AUTHOR OF THE ARTICLE

Filld Nor.4/7

SIR, On the 1st of May, 1871, an eruption took place on the north. west side of the island of Camaquin, killing many persons, destroying and sweeping away a whole village, and causing great destruction of property. During the months of February, March, and April the island had been subject to violent shocks of earthquake, which tore away sides of mountains and hurled huge boulders into the sea; this fortunate (?) circumstance caused thousands to leave the immediate vicinity, who otherwise would have been overwhelmed in the eruption.

On the evening of the 1st of May, at 6.30 p.m., at the foot of a mountain 5380 feet in height, and on a level plain, the earth belched forth fire, water, and huge masses of earth and stone. Ashes were carried to Zebu, a distance of seventy-seven miles, against the wind, then blowing over the sea (N.E.)-having no doubt being lifted into an upper current in that direction-and the houses, balconies, and streets were covered with ash. This first burst overwhelmed the village, destroyed all houses and plantations, and killed 150 persons, driving the remaining thousands to the islands of Bohul and Mindanao in terror, at the sacrifice of all their goods, &c.

From that time until now (July 11), when the Nassau visited it, it has smoked, and occasionally thrown out stones, &c., from its top and sides, raising a mound 400 feet in height of coarse brown scoria, strongly impregnated with sulphur, and having veins of metallic ore; here and there jets of steam and gas smelling strongly of sulphur, but of which it was impossible (without imminent risk) to get some in bottles for analysation.

It is not probable that another eruption will take place there, now that it has found vent. Steam appears to have been its origin, a great deal of it having been seen to issue; the water beneath the earth, having formed into steam by the heated earth, found an escape through the side of the hill or mountain. The depth at which this formed was no doubt great, as the stone thrown up is of great density, and had been subject to immense pressure. At a depth of three inches in the mass of scoria the finger could not bear the heat, and here and there jets of steam and smoke were issuing, as well as a sulphuric vapour.

Camaguin Island was the most productive in hemp of any of the group of Philippines. This hemp is peeled from the skin of the palm, much resem bling the common plantain or banana palm, but of lighter colour; it is combed out with a fine-toothed iron comb, dried, and sent to Manilla market, where it is exported in large quantities. The destruction, therefore, to this property, throwing thousands out of employment, is severely felt. A few, however, are gaining courage, and beginning to return to Camaguin.

I have not seen any notice of this in English papers, and therefore WILLIAM CHIMMO. send it, brief and imperfect as it is.

H.M.S. Nassau, surveying Sulu Sea, 1871.

SPECTROSCOPIC NOTES*

Test for Flatness of Surface. For testing the flatness of the prism surfaces, probably the best method is to focus a small

THE spectroscope consists essentially of three parts-a prism, or train of prisms, to disperse the light; a collimator, as it is called, whose office is to throw upon the prisms a beam of parallel rays coming from a narrow slit; and a telescope for viewing the spectrum formed by the prisms.

Supposing the slit to be illuminated by strictly homogeneous light, the rays proceeding from it are first rendered parallel by the object-glass of the collimator, are then deflected by the prisms and finally received upon the object-glass of the view-telescope, which, if the focal lengths of the collimator and telescope objectglasses are the same, forms at the focus a real image of the slit, its precise counterpart in every respect except that it is somewhat weakened by loss of light and slightly curved. +

If the focal length of the view-telescope is greater or less than that of the collimator, the size of the image is proportionally in. creased or diminished.

This image is viewed and magnified by the eye-piece of the telescope.

If now the light with which the slit is illuminated be composite, each kind of rays of different refrangibility will be differently reflected by the prisms, and form in the focus of the telescope its own image of the slit. The series of these images ranged side by side in the order of their colour constitutes the spectrum, which can be perfectly pure only when the slit is infinitely narrow (so that the successive images may not overlap), and accurately in the focus of the object-glass of the collimator, which object-glass, as well as that of the telescope, must be without aberration either chromatic or spherical, and the prisms must be perfectly homogeneous and their surfaces truly plane.

Of course, none of the conditions can be strictly fulfilled. An infinitely narrow slit would give only an infinitely faint spectrum ; and no prisms or object-glasses are absolutely free from faults. A reasonably close approximation to the necessary conditions can, however, be obtained by careful workmanship and adjust-❘ ment, and it becomes an important subject of inquiry how to adapt the different parts of the instrument to each other so as to secure the best effect, and how to test separately their excellence, in order to trace and remedy as far as possible all faults of performance.

With reference to the battery of prisms, several questions at once suggest themselves relative to the best angle and material, the number to be used, the methods of testing their surfaces and homogeneity, and the most effective manner of arranging them.

Angle and Material of the Prisms.-As to the refracting angle, the careful investigation of Prof. Pickering, published in the American Journal of Science and Art for May 1868, puts it beyond question that with the glass ordinarily employed an angle of about 60° is the best. For instruments of many prisms there is an advantage as regards the amount of light in making the angle such that the transmitted ray at each surface shall be exactly perpendicular to the reflected. For ordinary glass, the refracting angle determined by this condition somewhat exceeds 60°; for the so-called "extra-dense" flint it is a little less.

The high dispersive power of this "extra-dense" glass is certainly a great recommendation. But it is very yellow, powerfully absorbing the rays belonging to the upper portion of the spectrum, and is very seldom homogeneous. It is so soft also, and so liable to scratch and tarnish, that it can only be safely used by casing it with some harder and more permanent glass, as in the compound prisms of Mr. Grubb, and the direct vision prisms of many makers.

For many purposes these direct vision prisms are very convenient and useful, but they are hardly admissible in instruments of high dispersive power designed to secure accurate definition of the whole spectrum, the violet as well as the yellow.

* By C. A. Young, Ph.D., Professor of Natural Philosophy and Astronomy in Dartmouth College. Reprinted from advance-sheets of the Journal of the Franklin Institute, by permission of the Editor.

The curvature arises from the fact that the rays from the extremities of the slit, though nearly parallel to each other, make an appreciable angle with those which come from the centre. They therefore strike the surface of the prisms under different conditions from the central rays, and are differently refracted, usually more. The higher the dispersive power of the instrument and the shorter the focal length of the collimator, the greater this distortion, which is also accompanied by a slight indistinctness at the edges of the spectrum.

moon or some bright star), and then to scrutinise the image of the same object formed by reflection from the surface to be tested. Any general convexity or concavity will be indicated by a corresponding change of focus required in the telescope; any irregularity of form will produce indistinctness, and by using a cardboard screen perforated with a small orifice of perhaps & inch in diameter, the surface can be examined little by little, and the faulty spot precisely determined.

Test for Homogeneity.—It is not quite so easy to test the homogeneity of the glass. Any strong veins may, of course, be seen by holding the prism in the light, and if the ends of the prism are polished, the test by polarised light will be found very effective in bringing out any irregularities of density and elasticity in the glass. A blackened plate of window glass serves as the polariser; a Nicol's prism is held in one hand before the eye in such a position as to cut off the reflected ray, and with the other hand the glass to be tried is held between the Nicol and the polariser. If perfectly good it produces no effect whatever; if not it will show more or less light, usually in streaks and patches.

On the whole, however, the method of testing which has been found most delicate and satisfactory is the following:

A Geissler tube containing rarefied hydrogen is set up verti cally, and illuminated by a small induction coil.

A small and very perfect telescope of about six inches focus is directed upon it from a distance of seventy-five or one hundred feet, and carefully adjusted for distinct vision.

The prism to be tested is then placed in front of the objectglass of the telescope with its refracting edge vertical, adjusted approximately to the position of minimum deviation, and telescope and prism together then turned (by moving the table on which they stand), until the spectrum of the tube appears in the field of view. This spectrum consists mainly, as is well known, of three well-defined images of the tube, of which the red image, corresponding to the Cline, is the brightest and best defined, and stands out upon a nearly black background.

Supposing then the flatness of the prism surfaces to have been previously tested and approved, the goodness of the glass may be judged of by the appearance and behaviour of this red image; and by using a perforated screen in the manner before described, inequalities of optical density are easily detected and located. Irregularities, which would hardly be worth noticing in a telescope object-glass, where the total deviation produced by the refraction of the rays is so small, are fatal to definition in a spectroscope, especially one of many prisms, and it is very difficult to find glass which will bear the above-named test without flinching. Of course it must be conducted at night, or in a darkened room.

Number and Arrangement of Prisms.-The number of prisms to be employed will depend upon circumstances. If the spectrum to be examined be faint, and either continuous or marked with dark lines, or by diffuse bands, either bright or dark, we are limited to a train of few prisms.

The light of the sun is so brilliant that, in studying its spectrum, we may use as many as we please. The light is abundant after passing through 13, and I presume would still be so if the train were doubled.

Spectra of fine well-defined bright lines also bear a surprising number of prisms. The loss of light arising from the transmission through many surfaces is nearly, if not quite, counterbalanced by the increased blackness of the background, and the greater width of slit which can be used.

As to the best arrangement for the prisms, this also must be determined by circumstances.

Where exact measurements are aimed at, as, for instance, for the purpose of ascertaining the wave-length of lines, or the dispersion co-efficient of a transparent medium, the prism or prisms ought to be firmly secured in a positive and determinable relation to the collimator. A train of many prisms can hardly be safely used in such work on account of the difficulty in obtaining this necessary fixity, and if high dispersion is indispensable, it can only be obtained by enlarging the apparatus.

But for most purposes it is better that the prisms, instead of being fixed, should be mounted upon some plan which will secure their automatic adjustment to the position of minimum deviation.

Having now thoroughly tried the plan which I proposed and

published in this Journal last November, I am prepared to say that I cannot imagine anything more effective and convenient.

The arrangement of Mr. Browning and its extension by Mr. Proctor, are equally effective so far as the adjustment of the prisms is concerned, but are less compact and simple, and do not afford the same facility in changing the number of prisms in

use.

In my instrument the light, after leaving the collimator, falls perpendicularly upon the face of a half-prism, passes through the train

of the transmitted beam. In other words a prism of the same material and angle described, in order to transmit a beam one inch in diameter, must be one inch high and have sides 1 inches long.

But when the light is received perpendicularly upon the face of a half prism, as in Fig. 3, then, since be be÷cos 30°, the length of the prism side, bc, requires to be only 1155 times as great as the diameter of the transmitted beam.

Thus a train of prisms each 1 inch high, and having the sides of their triangular bases each 1155 inches long, led by an initial half prism in the way indicated, would transmit a beam I inch in diameter, while without the initial half prism the sides would

Fig. 1.

TELESCOPE

of prisms near their bases; at the end of the train is twice totally reflected by a rectangular prism attached to the last of the train (which is also a half prism), is thus transferred to the upper story of the train, so to speak, and returns to the view-telescope, which is firmly attached to the same mounting as the collimator and directly above it. Both are immovable, and the different portions of the spectrum are brought into view by means of the screw, which acts upon the last prism, and through it upon the whole train. The adjustment for focus is by a milled head, which carries the object-glasses of both collimator and telescope in or out together. Since they have the same focal length, this secures the accurate parallelism of the rays as they traverse the prisms. The annexed diagram, taken from the paper already alluded to, exhibits the plan of the arrangement, and requires no explanation, unless to add that, to avoid complication in the figure, I have represented only two of the radial forks which maintain the prisms in adjustment; also, that the prisms are connected to each other at top and bottom, not by hinges, but by flat springs, preventing all shake.

*

By adding another tier of prisms and sending the light back and forth through a third and fourth story, the dispersion can be easily doubled with very small additional expense, except for the prisms themselves; the mechanical arrangements remaining precisely the same.

I desire, in this connection, to call attention to the great ad

have to be 1667 long, the surface to be worked and polished would be 144 (i e. 1667-1155) times as great, and the quantity of glass required 2'08 (i.e. 1'442) times as great. With a higher index of refraction the gain is still greater.

This advantage of course is not obtained without losing the dispersive power of one half prism. But where the train is extensive this loss is comparatively insignificant, and may be made up by a slight increase of the refracting angles. Indeed, in an instrument of the form above described, it is necessary, if the train is led by a whole prism, to reduce the refracting angle from 60° to about 55°, in order that the reflecting prism at the end of the train may not interfere with the collimator, while with the initial half prism the full angle of 60° can be used, so that in this case there is practically no loss whatever.

It would seem to deserve consideration, whether in the construction of spectroscopes to be used with some of the huge telescopes now building, it may not be advisable to carry the principle still further, by employing two or more half prisms at the beginning of the train in order to economise material and weight.

Dispersive Efficiency.-The dispersive efficiency of the spectroscope is its ability to separate and distinguish spectral lines whose indices of refraction differ but slightly; it is closely analogous to the dividing power of a telescope in dealing with double stars. It depends not only upon the train of prisms, but also upon the focal lengths of the telescope and collimator, the width of the slit, and the magnifying power of the eye-piece.

As has been said before, each bright line is an image of the

notice.

vantages gained by the use of the half prism at the commencement of the train, a point which hitherto seems to have escaped With a prism of 60°, having a mean refractive index, μ, 16, and placed in its best position, the course of the rays is as shown in Fig. 2. The side a b is just 13 times the cross section, a d,

*After the appearance of the article referred to, I found that Mr. Lockyer had anticipated me by some months, not only in respect to the method of making the rays traverse the prism train twice, but also in the use of a half prism at the beginning of the train, and the employment of an elastic spring in the adjustment for minimum deviation. In all essential particulars his instrument is the same as mine, though in some matters of detail there are differences which have proved to be of practical importance in favour of

slit whose magnitude, referred to the limit of distinct vision, depends upon the telescope and collimator, but is independent of the prism train. The distance between the centres of two neighbouring lines, on the other hand, depends upon the number and character of the prisms, the focal length of the telescope, and the magnifying power of its eye-piece, but is totally inde. pendent of the collimator.

In order that two lines may be divided, it is necessary that the edges of their spectral images should be separated by a certain small distance-a minimum visibile, whose precise value is of no particular importance to our present purpose, but which I suppose to be about of an inch.

*It is very common to describe the dispersive power of a spectroscope as being equivalent to a certain number of prisms, or a certain number of degrees from A to H. But either method fails entirely to convey an idea of the appearance of the spectrum in the instrument, and it is much better to name the closest double line which it will divide, or else to give the distance between the two D lines, either linear (referred of course to the limit of distinct vision), or angular. If we know, for example, that the D lines are separated, or, what comes to the same thing, appear to be one-sixth of an inch apart, we have a definite idea of the power of the instrument.