by Mr. Sorby in the May number of this Journal, and by Mr. Browning in 1867, I have observed clearly in the living plant, accompanied by the strong line of chlorophyl in the red, which also forms part of the plant's colour; and of the two lines of Phycocyan, that at 41, in the green (characteristic of Mr. Sorby's red constituent), was the stronger.

Cohn describes his Phycocyan as being the blue colouring matter of all the Phycochromacea, which includes many genera besides Oscillaria. It is possible that among these genera there may be other colouring matters, but it is not impossible that variations are caused by the preponderance of either the red or blue element of Sorby, and this preponderance may vary with the season or other conditions. The spectra from Peltigera, Collema, and Oscillaria, given by Askenasy, are all much alike, and differ chiefly in the relative intensities of the two bands; Mr. Sheppard's fluid and Cohn's Phycocyan are equally close, but the colouring matters of Peltigera and Collema are likely enough distinct from that of Oscillariæ, though this has to be proved. The want of some universal standard for indicating spectra is strongly seen in the difficulty there is in comparing the statements of these authors. The Phycocyan of Cohn, and the colour described by Sheppard, Browning, and Sorby, are so similar in properties and mode of occurrence that it appears probable that the small differences in their spectra, as figured by Cohn and Browning, may be explained away. Whether identical or not, they are so closely allied that it will be well to speak of Mr. Sheppard's as the Phycocyan of Sheppard, and Cohn's as the Phycocyan of Cohn; it being understood that, as shown by Sorby, a red and a blue constituent may be distinguished in these.

I trust I have not made this communication too long, the purpose of it being to remove the misapprehension to which your remarks in the annual address relative to Mr. Sorby's views and my own might give rise, and I hope that you will now reconsider my ecclesiastical deposition, and if not endow, at any rate re-establish the doctrine of

Yours very truly,

E. RAY LANKESTER.

Note. The President, in forwarding this letter for insertion, desires us to state that the establishment of the truth will leave no room for Mr. Lankester's assertion that Mr. Sheppard has "quite unnecessarily conjured up a mystery." An extract from Mr. Glaisher's Presidential Address, published in April, 1868, gives Dr. Cohn's view of the mystery in question:-"Dr. Cohn, in thanking Mr. Sheppard for his highly-interesting communication,' admits the necessity of further experiments, 'that the truth may be established;' and after intimating his intention to pursue the subject further, he concludes, 'I shall also endeavour to repeat your experiments with albumen, the influence of which upon the colour seems very curious after your investigations.""-ED. M. M. J.

VOL. IV.

Journal, July

III.-Object-glasses and their Definition.

By F. H. WENHAM, Vice-President R.M.S.

As this subject is now under discussion, I again venture to trespass on these pages with a few remarks. If I incline to defend our highest powers against the assertion that "the best of them produce spherical aberrations," it is not because I am disposed to let well alone, and rest under the personality of a "satisfied optician," but on the plea that both the assumed bead structure of the Podura, and also the optical means resorted to for demonstrating such an error, have failed in proving its existence.

By the use of the plane mirror and sunlight, Colonel Dr. Woodward at once cleverly hits upon the illumination that affords an analysis of this beaded appearance, and considers it as an interference phenomenon. That this gentleman possesses object-glasses of the first quality, with the most unrivalled skill in adjusting and using them, cannot be questioned, as his superb photographs of difficult testobjects testify. Having seen the beaded appearance, his extreme diffidence about obtaining a photograph of it is not very encouraging for the advocates of this structure, and Dr. W. suggests that Dr. Maddox be induced to undertake the task. If this structure is to be generally admitted it will be by the aid of the skill of these gentlemen, but I much doubt whether it can be accomplished, for there is a truthfulness in the photographs of these tests that scarcely admits of a deception. The most difficult markings will come out, almost in spite of the illumination, which does not even require that nicety of adjustment sometimes so tedious in ordinary observation.

In November, 1854, I read a paper "On Microscopic Photography" before the Microscopical Society. I had previously obtained impressions of several test-objects, and exhibited one of the Angulatum magnified 15,000 diameters, and stated at the time that it "shows the configuration of the markings perfectly black and distinct, in a far greater degree than we can ever hope to see them through the compound microscope, and it is my opinion that if ever the structure of these difficult tests is to be proved it will be by the aid of photography." This prediction has since been verified by the labours of Drs. Maddox and Woodward.

The assertion that "if a blaze of light is sent through the microscope, false appearances are exhibited," must need some qualification. I have condensed the rays from the clear meridian sun, first with a bull's-eye three inches in diameter, and then through an achromatic condenser, upon the Rhomboides and other difficult tests, which are thus enveloped in a fearful blaze of light and heat, but gloriously does the picture emerge at last with the most delicate markings shown on a screen 10 feet distant. This is unquestion

ably a test for object-glasses of good quality and workmanship, for not only may chromatic and spherical errors be detected, but any scratches or particles of dust on the surface of the lenses may be mapped out.

It was remarked by Mr. Slack (who may be presumed to be familiar with the illumination employed), "that the effect produced by Dr. Pigott had no connection whatever with the mode of illumination adopted by Dr. Woodward." As the question at present stands, such a defence is easy, for I in common with many others may well reiterate the question, What is Dr. Pigott's method of illumination? The directions given in the paper of November 10th, 1869, are so vague and scanty, that many having the best glasses are unable to develop the beaded structure, and those that do obtain it are compelled to find out a method for themselves as Dr. Woodward has done.

It is scarcely to be expected that those who have not in some degree been practically familiar with the construction of objectglasses, can be fully aware of the value of the mercury globule in originating combinations. To the optician it is as needful as the callipers and straight-edge to the engine-fitter-every glass is separately tested by it. Its familiar readings show whether the work is going on right or wrong; by the indications of inward or outward coma whether the oblique pencils are correct, and finally, the least chromatic or spherical error can be ascertained by its means. It may be "well-known to mathematicians that these globules are not perfectly spherical" (and mathematicians will be correct in all things), but setting aside the fact that the more minute the particle the nearer it approaches to a true sphere, it happens that shape is not of the smallest consequence, or whether it is illuminated by oblique light, for it is not the globule but the absolute point of light reflected from it that is used. The diameter of a mercury globule for correcting the highest powers from a upwards is only the one five thousandth part of an inch. Perhaps some one who thinks it may advance the subject, will be good enough to calculate the size of the image of a small lamp-flame set at 4 inches distance, reflected from the surface of a convex mirror of 10,000 of an inch radius.

Dr. Pigott, by converting the microscope object-glass into a species of telescope, and viewing distant and minute discs of light, professes by means of the "Aplanatic Searcher"!!! to have discovered spherical error in all our best glasses, to the existence of which everyone else has hitherto been blind. Doubtless a very imposing or "striking" demonstration may be made out of this, but it is easy to demonstrate that by so doing we are setting at naught the very qualities and advantages of large angular pencils. The conjugate foci are now so far distant, that large angular aperture no longer exists. A difference in the adjusting collar that would pro

duce an enormous amount of spherical aberration, when the objectglass is tried on the globule test, or in its legitimate use as a microscope lens, is scarcely perceptible in the telescope arrangement, and though a badly-corrected glass may not form an image, yet I have no hesitation in affirming that a lens may be made to give perfect definition under the latter condition, that will prove utterly worthless as an objective for the microscope.

IV.-On the Optical Advantages of Immersion Lenses and the Use of Deviation Tables for Optical Research. By ROYSTONPIGOTT, M.A., M.D., M.R.C.P., F.C.P.S., F.R.A.S., late Fellow of St. Peter's College, Cambridge.

Introduction.

ONE might almost venture to declare that perhaps too much exclusive attention has been paid to objectives to the neglect of the pencils radiating from the object. A brilliant particle, as a podura or diatom- spherule, throws out a divergent pencil, which suffers extraordinary deviations and reflexions before it is permitted to enter the object-glass at all. The infinitely small here escapes our attention. But the primary behaviour of the tiny spray of rays is of the last importance to the final definition, according to the refractions, dispersions and reflexions it undergoes in its primary or nascent state, if the term be allowed, of the diverging pencils. Again, immersed in a refracting fluid and again covered, new transformations into fresh strands of the divergent pencils impose new optical conditions. A large proportion of the rays are absolutely refused admittance by the covering glass. But some of these rejected rays are at once permitted to pass into the objective by the intervention of fluid-films instead of air.

The tracing of the rays (to scale) for many different selected divergent rays emanating from the brilliant particle, and the investigation of their behaviour under the different resistances offered by interposed media of different deflecting powers, is a problem worthy of a great deal of hard labour. The right understanding of this primary process, among the innumerable rays just about to enter the object-glass, nascent as it were from their very source of light, may possibly lead to some new and beautiful results at present unexpected.

The student of these nascent rays, however, must be prepared for some laborious work at the logarithmic tables of numbers, and sines of the divergent angles, and calculate accurately the mutual refractive indices between the different substances and each other.

At present, after much opposition in this country, the method (so much adopted abroad, in Europe and America) of immersion lenses has gained a remarkable footing, water being employed between the objective and the covering glass.

I propose in the following papers to give an account of some researches in this difficult subject, which I hope may tend to stimulate others to more successful exertions than

my own.

And to introduce the reader at once to the subject-matter, I will just mention here some results already obtained, which seem to repay the trouble of making the tables.

The greatest aperture of a pencil divergent from a brilliant particle immersed in Canada balsam and covered with a thin plate-glass cover, is only

2 x 41° 48' or 93° 36'.

When water is substituted for air this is enlarged to

2 x 62° 57' or 125° 54'.

If a flint-glass cover could be made, the divergent pencil admitted to the object-glass when oil of turpentine is the immersion fluid would mount up to

2 x 78° 37' or 157° 14'.

Another point of most essential importance is the actual deviation of the pencils. The great desideratum is a minimum deviation for all pencils, and therefore a greater simplification of the refractions. On this point, to tempt the reader to attentively consider the subject, I will venture to assure him that the quantity of light and the chromatic dispersion of the divergent pencils of the object is wondrously changed according to the optical nature of the film-be it in water or other fluid-interposed between the objective and the covering glass, though the thickness of this delicate plate may not exceed half a hundredth of an inch. Thus it will be found that calling the quantity of light gained by a given dry or pneumo-objective unity, that for other arrangements it may be thus arranged :

Once more; I am about to discuss the amount of deviation for different media from 1° to 90° of incidence, occupying this minute yet interesting interval. That the deviation for moderate angles is three times less for the water lens than for the dry is a novel finding which should stimulate inquiry. It is probable that the means really at our disposal for improving even our best glasses are very far from being exhausted. Let not our motto be, "It is enough, rest and