radiated from it due to such matter at a high temperature. The nebular hypothesis of Laplace at the end of the last century required, indeed, that matter similar to that of the earth should exist throughout the solar system; but then this hypothesis itself needed for its full confirmation the independent and direct observation that the solar matter was terrestrial in its nature. This theoretical probability in the case of the sun vanished almost into thin air when the attempt was made to extend it to the stellar hosts; for it might well be urged that in those immensely distant regions an original difference of the primordial stuff as well as other conditions of condensation were present, giving rise to groups of substances which have but little analogy with those of our earthly chemistry.

The dark lines were described first by Wollaston in 1792, who strangely associated them with the boundaries of the spectral colors, and so turned contemporary thought away from the direction in which lay their true significance. It was left to Fraunhofer in 1815, by whose name the dark lines are still known, not only to map some 600 of them, but also to discover similar lines, but differently arranged, in several stars. Further, he found that a pair of dark lines in the solar spectrum appeared to correspond in their position in the spectrum, and in their distance from each other, to a pair of bright lines which were nearly always present in terrestrial flames. This last observation contained the key to the interpretation of the dark lines as a code of symbols, but Fraunhofer failed to use it; and the birth of astrophysics was delayed. An observation by Forbes at the eclipse of 1836 led thought away from the suggestive experiments of Fraunhofer; so that in the very year of the Queen's accession the knowledge of the time had to be summed up by Mrs. Somerville in the negation: "We are still ignorant of the cause of these rayless bands."

Later on the revelation came more or less fully to many minds. Foucault, Balfour, Stewart, Ångström prepared the way. Prophetic guesses were made by Stokes and by Lord Kelvin. But it was Kirchhoff who, in 1859, first fully developed the true significance of the dark lines; and by his joint work with Bunsen on the solar spectrum proved beyond all question that the dark lines in the spectrum of the sun are produced by the absorption of the vapors of the same substances, which when suitably heated give out corresponding bright lines; and, further, that many of the solar absorbing vapors are those of substances found upon the earth. The new astronomy was born.

Soon after the close of 1862, in collaboration with Dr. W. A. Miller, I sent a preliminary note to the Royal Society, "On the lines of some of the fixed stars," in which we gave diagrams of the spectra of Sirius, Betelgeux, and Aldebaran, with the statement that we had observed the spectra of some 40 stars, and also the spectra of the planets Jupiter and Mars. It was a little remarkable that on the same day on which our paper was to be read, but some little time after it had been sent in, news arrived there from America that similar observations on some of the stars had been made by Mr. Rutherford. A very little later similar work on the spectra of the stars was undertaken in Rome by Secchi and in Germany by Vogel.

In February, 1863, the strictly astronomical character of the observatory was further encroached upon by the erection, in one corner, of a small photographic tent furnished with baths and other appliances for the wet collodion process. We obtained photographs, indeed, of the spectra of Sirius and Capella; but from want of steadiness and more perfect adjustment of the instruments, the spectra, though defined at the edges, did not show the dark lines as we expected. The dry collodion plates then available were not rapid enough; and the wet process was so inconvenient for long exposures, from irregular drying, and

draining back from the positions in which the plates had often to be put, that we did not persevere in our attempts to photograph the stellar spectra. I resumed them with success in 1875, as we shall see further on.

Whenever the nights were fine, our work on the spectra of the stars went on, and the results were communicated to the Royal Society in April, 1864; after which Dr. Miller had not sufficient leisure to continue working with me. * I pass on at once, therefore, to the year 1876, in which, by the aid of the new dry plates, with gelatin films, introduced by Mr. Kennett, I was able to take up again, and this time with success, the photography of the spectra of the stars, of my early attempts at which I have already spoken.

By this time I had the great happiness of having secured an able and enthusiastic assistant by my marriage in 1875.

The great and notable advances in astronomical methods and discoveries by means of photography, since 1875, are due almost entirely to the great advantages which the gelatin dry plate possesses for use in the observatory over the process of Daguerre, and even over that of wet collodion. The silverbromide gelatin plate, which I was the first, I believe, to use for photographing the spectra of stars, except for its grained texture, meets the need of the astronomer at all points. This plate possesses extreme sensitiveness; it is always ready for use; it can be placed in any position; it can be exposed for hours; lastly, immediate development is not necessary, and for this reason, as I soon found to be necessary in this climate, it can be exposed again to the same object on succeeding nights; and so make up by successive installments, as the weather may permit, the total long exposure which may be needful.

The power of the eye falls off as the spectrum extends beyond the blue, and soon fails altogether. There is, therefore, no drawback to the use of glass for the prisms and lenses of a visual spectroscope. But while the sensitiveness of a photographic plate is not similarly limited, glass, like the eye, is imperfectly transparent, and soon becomes opaque, to the parts of the spectrum at a short distance beyond the limit of the visible spectrum. To obtain, therefore, upon the plate a spectrum complete at the blue end of stellar light, it was necessary to avoid glass and to employ instead Iceland spar and rock crystal, which are transparent up to the limit of the ultra-violet light which can reach us through our atmosphere. Such a spectroscope was constructed and fixed with its slit at the focus of the great speculum of the Cassegrain telescope.

How was the image of a star to be easily brought, and then kept, for an hour, or even for many hours, precisely at one place on a slit so narrow as about the one two-hundredth of an inch? For this purpose the very convenient device was adopted of making the slit-plates of highly polished metal, so as to form a divided mirror, in which the reflected image of a star could be observed from the eye end of the telescope by means of a small telescope fixed within the central hole of the great mirror. A photograph of the spectrum of a Lyræ, taken with this instrument, was shown at the Royal Society in 1876.

In the spectra of such stars as Sirius and Vega, there came out in the ultraviolet region, which up to that time had remained unexplored, the completion of a grand rhythmical group of strong dark lines, of which the well-known hydrogen lines in the visible region form the lower members. Terrestrial chemistry became enriched with a more complete knowledge of the spectrum of hydrogen from the stars. Shortly afterwards, Cornu succeeded in photographing a similar spectrum in his laboratory from earthly hydrogen.

The years 1863 to 1890 were made fruitful by Huggins, especially in the comparison of terrestrial and stellar spectra. He established that the principal elements in the earth's surface strata exist also in the atmospheres of the stars in the form of vapors and gases. Other studies attempted to arrange the principal stars in the order of their evolutionary history-in the order of their effective ages-from the different appearances of the hydrogen and metallic lines in their spectra.

Huggins's observation of the spectrum of a nebula, for the first time in 1864, has probably never been surpassed in dramatic interest in any department of science. From the days of Sir William Herschel it had been a much-discussed question whether the nebulathe faintly shining bodies which had not been resolved into separate star images—were continuous in structure like a great gaseous cloud, or were composed of stars unresolvable on account of their enormous distances. To let Huggins speak:

The nature of these mysterious bodies was still an unread riddle. Toward the end of the last century the elder Herschel, from his observations at Slough, came very near suggesting what is doubtless the true nature and, place in the cosmos, of the nebulæ. I will let him speak in his own words:

"A shining fluid of a nature unknown to us.

66

What a field of novelty is here opened to our conceptions!

We

may now explain that very extensive nebulosity, expanded over more than 60° of the heavens, about the constellation of Orion, a luminous matter accounting much better for it than clustering stars at a distance.

"If this matter is self-luminous it seems more fit to produce a star by its condensation than to depend on the star for its existence."

This view of the nebulæ as parts of a fiery mist out of which the heavens had been slowly fashioned, began, a little before the middle of the present century, at least in many minds, to give way before the revelations of the giant telescopes which had come into use, and especially of the telescope, 6 feet in diameter, constructed by the late Earl of Rosse at a cost of not less than £12,000.

Nebula after nebula yielded, being resolved apparently into innumerable stars, as the optical power was increased; and so the opinion began to gain ground that all nebula may be capable of resolution into stars. According to this view, nebule would have to be regarded, not as early stages of an evolutional progress, but rather as stellar galaxies already formed, external to our system-cosmical "sandheaps"-too remote to be separated into their component stars. Lord Rosse himself was careful to point out that it would be unsafe from his observations to conclude that all nebulosity is but the glare of stars too remote to be resolved by our instruments. In 1858 Herbert Spencer showed clearly that, notwithstanding the Parsonstown revelations, the evidence from the observation of nebulæ up to that time was really in favor of their being early stages of an evolutional progression.

On the evening of August 29, 1864, I directed my telescope for the first time to a planetary nebula in Draco. The reader may now be able to picture to himself to some extent the feeling of excited suspense, mingled with a degree of awe, with which, after a few minutes of hesitation, I put my eye to the spectroscope. Was I not about to look into a secret place of creation?

I looked into the spectroscope. No spectrum such as I expected. A single bright line only. At first I suspected some displacement of the prism, and that I was looking at a reflection of the illuminated slit from one of its faces. This thought was scarcely more than momentary. Then the true interpretation flashed upon me. The light of the nebula was monochromatic, and so, unlike any other light I had as yet subjected to prismatic examination, could not be extended out to form a complete spectrum. After passing through the two prisms it remained concentrated into a single bright line, having a width corresponding to the width of the slit, and occupying in the instrument a position at that part of the spectrum to which its light belongs in refrangibility. A little closer looking showed two other bright lines on the side toward the blue, all the three lines being separated by intervals relatively dark.

The riddle of nebula was solved. The answer, which had come to us in the light itself, read: Not an aggregation of stars, but a luminous gas. Stars after the order of our own sun, and of the brighter stars, would give a different spectrum; the light of this nebula had clearly been emitted by a luminous gas. With an excess of caution, at the moment I did not venture to go further than to point out that we had here to do with bodies of an order quite different from that of the stars. Further observations soon convinced me that, though the short span of human life is far too minute relatively to cosmical events for us to expect to see in succession any distinct step in so august a process, the probability is, indeed, overwhelming in favor of an evolution in the past, and still going on, of the heavenly hosts. A time surely existed when the matter now condensed into the sun and planets filled the whole space occupied by the solar system, in the condition of gas, which then appeared as a glowing nebula, after the order, it may be, of some now existing in the heavens. There remained no room for doubt that the nebulæ, which our telescopes revealed to us, are the early stages of long processions of cosmical events, which correspond broadly to those required by the nebular hypothesis in one or other of its forms.

Further observations identified two of the lines as due to hydrogen. Observations by various spectroscopists have increased the number of bright lines known to exist in nebular spectra to 30 or 40, but aside from hydrogen and helium, accounting for about one-half of all the observed lines, the chemical constitution of the so-called gaseous nebulæ is unknown.

To leave the subject of the nebular spectrum here would mislead the inexperienced, and it is necessary to say that only a minority of the nebulæ thus far observed in this way show spectra consisting chiefly of bright lines. The spiral nebulæ have spectra chiefly continuous, and their composition and physical state remain a mystery. Even so for bright-line nebulæ, as observed by Huggins in 1864, we cannot say that they are shining by virtue of the heat of incandescence; the tendency of present-day opinion is that their substances are comparatively cool, and that their luminosity must arise from other conditions not now understood with certainty.

Important contributions to our knowledge of temporary stars-the so-called new stars-were made by Huggins in half a dozen papers on their spectra. The principal stars studied were those which ap

peared suddenly in Corona Borealis, in the Great Nebula in Andromeda, and in Auriga.

Huggins was among the first to apply the spectroscope to the study of comets. A dozen papers by him, on cometary spectra, make interesting reading, for they record the gradual evolution of our knowledge of physical conditions existing in comets up to the year 1882. For example, speaking of observations of Winnecke's comet of 1868 made on the evening of June 22, he says:

When a spectroscope furnished with two prisms of 60° was applied to the telescope, the light of the comet was resolved into three very broad, bright bands. *

*

In the two more refrangible of these bands the light was brightest at the less refrangible end, and gradually diminished toward the other limit of the bands. This gradation of light was not uniform in the middle and brightest band, which continued of nearly equal brilliancy for about one-third of its breadth from the less refrangible end. This band appeared to be commenced at its brightest side by a bright line.

The least refrangible of the three bands did not exhibit a similar marked gradation of brightness. This band, though of nearly uniform brilliancy throughout, was perhaps brightest about the middle of its breadth.

*

The following day I carefully considered these observations of the comet with the hope of a possible identification of its spectrum with that of some terrestrial substance. The spectrum of the comet appeared to me to resemble some of the forms of the spectrum of carbon which I had observed and carefully measured in 1864. On comparing the spectrum of the comet with the diagrams of these spectra of carbon, I was much interested to preceive that the positions of the bands in the spectrum, as well as their general characters and relative brightness, agreed exactly with the spectrum of carbon when the spark is taken in olefiant gas. **

**

It was with the spectrum of carbon, as thus obtained, that the spectrum of the comet appeared to agree. It seemed, therefore, to be of much importance that the spectrum of the spark in olefiant gas should be compared directly in the spectroscope with the spectrum of the comet. The comparison of the gas with the comet was made the same evening, June 23. *

The brightest end of the middle band of the cometic spectrum was seen to be coincident with the commencement of the corresponding band in the spectrum of the spark. As this limit of the band was well defined in both spectra, the coincidence could be satisfactorily observed up to the power of the spectroscope; and may be considered to be determined within about the distance which separates the components of the double line D. As the limits of the other bands were less distinctly seen, the same amount of certainty of exact coincidence could not be obtained. We considered these bands to agree precisely in position with the bands corresponding to them in the spectrum of the spark.

The apparent identity of the spectrum of the comet with that of carbon rests not only on the coincidence of position in the spectrum of the bands, but also upon the very remarkable resemblance of the corresponding bands in their general characters and in their relative brightness. This is very noticeable in the middle band, where the gradation of brightness is not uniform. This band in both spectra remained of nearly equal brightness for the same proportion of its length.