Pierre Jules César Janssen

“ The priceless advantages which photography offers for the prosecution of our experiments are, in short, the preservation of the images, the extension of sensibility, and the faculty of seizing phenomena of the most different degrees of illumination, including the extremely strong and the extremely weak.
The above is a very incomplete picture of what has been accomplished by physical astronomy. Is it not enough, however, to show that our branch of the science has already attained the height of its elder sister? ”

“ So the whole visible universe—not only our central star and the planets of our family, but those stars, too, which are so far off that our most powerful telescopes can not give them a sensible diameter, and those nebulæ which only make a weak glow in our instruments—is reached by our chemistry, seized by our analysis, and made to furnish the proof that all matter is one, and that these stars are made of the same stuff as we. ”

“ Chemistry here comes to the aid of mechanics, and we may say boldly that that matter, which is identical with ours, is subject, like it, to the laws of gravitation. Certainly, when Newton decomposed a beam of white light, and laid the first basis of the theory of the spectrum, he had not the slightest suspicion that his law of gravitation would, at a later period, find in it wings to carry it into regions where all measurement ceases and all calculation is powerless. ”

“ It must be added that the chemical nature of a body is not the exclusive element in the constitution of its spectrum, but that that may vary with the physical circumstances of the phenomenon, the temperature, the cause generating the radiation, etc.; but these are subordinate effects, which only add to the richness of the method, without detracting from its certainty and its value. ”

“ These discoveries belong pre-eminently to physical astronomy, and form its first courses. If we except the sun and moon, which have a very sensible diameter, and admit of some observations without the aid of the telescope, all the stars appear to the eye only as brilliant points, and admit of no studies except of their motions. Therefore, an astronomy without the telescope would never have permitted us otherwise than as a matter of probability to consider the planets as like the earth in form, constitution, and office. ”

“ It is proper to remark at this point that, when we analyze light in this way to examine it in its elements, we perform an operation entirely parallel to that of the chemist who separates the simple elements of a compound body. The elementary ray is a chemical species in light. It has all the characteristics of a species. It is incapable of decomposition, it has an individuality of its own, characterized by its wave-length, by the physiological effects it induces, whether acting alone or in association with other rays, and by the phenomena which it exhibits in its relations with bodies. ”

“ These observations and experiments doubled the field of investigation opened to spectrum analysis. Not only could the incandescent atmospheres of the sun and the stars now be made to reveal their nature and their composition to us, but our researches might also be extended to objects having a still greater interest for us. We could at once take our own atmosphere for an object, investigating high and inaccessible regions, and making analyses in them which could not be attempted by any other means. ”

“ The little mass of matter forming the chemical molecule, when it can vibrate freely, as in the gaseous state, emits a peculiar system of waves, a system which varies principally with the chemical species of the molecule, but which varies also, though rather secondarily, with the distance apart of the molecules and the nature and intensity of the forces that induce a vibratory movement in it. ”

“ Are not the two worthy of each other, and will they not be able to march hereafter at an equal pace to the conquest of the heavens? On one side we behold the calculus—that marvelous intellectual lever, putting the data of observation to work, and drawing from them magnificent and unexpected consequences. ”

“ We should, then, need a telescope magnifying more than two hundred thousand times to show us, under the most favorable circumstances, a star's diameter equal to that which the sun presents to the naked eye. Such a power is a hundred times greater than the strongest powers that can be used. ”

“ Then, going away from the earth, we could interrogate the planetary atmospheres, and seek in them the vapor of water, and with it one of the first conditions of the development of terrestrial life. We could also, comparing the composition of the planetary atmospheres with the astronomical facts which permit us to judge of the geological conditions of the surfaces of the planets, follow in them the atmospheric evolutions which on the earth belong to the domain of the past or of the future. ”

“ We are, therefore, obliged to stay within the limits of our system, and proceed by analogy when we desire to go out of it. The analogies, it is true, were very powerful means even with Copernicus and Galileo, but with Kirchhoff and Huggins they were destined to acquire very shortly an irresistible force. Nature habitually reserves for the assiduous and sagacious observer surprises that exceed his hopes. So, while the study of the stars, considered as individual worlds, still remained beyond our reach, a great observer discovered facts of a very general bearing. ”

“ We have been able to leap over the enormous distance which separates the conception of the body, viewed as to its general properties, from the notion of it as individualized in such a manner as to constitute a chemical species by regarding light not only as a whole, but also in its elementary parts; by not only studying the whole beam and the general modifications that affect it, but by extending the examination to the elementary rays of which it is composed. ”

“ I refer to the telescope, spectrum analysis, and photography.
The foundations of physical astronomy were laid in the invention of the telescope. Every one has heard of the emotion which filled Europe at the announcement of the discovery of an instrument which had the power of making distant objects appear as if they were near. ”

“ On one side behold, finally, the heavens measured, the solar world placed in the balance, and its movements so well linked together by the law that governs them that soon, perhaps, past, present, and future will no longer exist for astronomy. ”

“ This discovery had an immense philosophical bearing, for it proved that the matter forming the solar and the stellar world is obtained from the same elements. It was a demonstration of the material unity of the universe. The study was prosecuted still further. There are stars which we regard as situated on the confines of the visible universe, the light of which is so weakened by the immense journey it has to make to reach us that they appear only as feeble glows. ”

“ It is an admirable artifice, which in a certain manner prevents the extinction of the phenomenon and its passage to among the things that were, and keeps it always with us for examination or study. Important as these results may be, the latest labors of which photography has been the object, especially in what concerns the sun, have demonstrated that the method may be employed as a means of making discoveries in astronomy. ”

“ I shall endeavor to present to you a picture, sketched in broad outline, of the progress and influence of a branch of research which has played a considerable part in contemporary scientific movements, and the discoveries of which have not only revolutionized our astronomical knowledge, but have also opened new and unexpected horizons to philosophy—I mean physical astronomy. ”

“ The images of any object impressed by light upon the eye are fugacious, and can be of only a limited intensity. The images fixed upon the photographic plate are permanent, and can be made of an intensity that becomes cumulative with the duration of the exposure. The photographic retina may be expected, when the art has been perfected in the highest degree, to give us images corresponding with an extremely expanded range in the duration of the exposure. ”

“ This great idea of the specific character of luminous rays is due to Newton. It was introduced into science when that great genius gave his explanation of the action of the prism on white light. The foundations of spectrum analysis were laid at that time, and the study of it might have been begun then at once; but the human mind does not proceed with so penetrating and absolute a logic as that. ”

“ The planetary stars are not, however, the only ones that lend themselves to these applications. There are also fixed stars the spectra of which reveal the characteristics of the vapor of water. Now, we know that the atmosphere of a star must be considerably cooled to permit the gases of which water is constituted to combine and generate a vapor. Our sun is still very far from this critical condition. ”

“ They have given a body of life to what was slumbering in posse; they have thus shown themselves worthy continuers of the work of Newton.
You know that this spectrum analysis made its appearance very suddenly in science. You may recollect the emotion that affected us all when it was announced that the solar atmosphere had been chemically analyzed, and a list of the metals it contained was published. You are, however, too well acquainted with the history of science to suppose that a method as complete as the one that was thus announced had no antecedents. ”

“ The large solar images which have been obtained in the latest years at Meudon have revealed phenomena of the surface of the sun which our largest observatory instruments could not have shown, and which open a new field of studies. By their aid we can at last distinguish the real form of those elements of the photosphere, respecting which so many different and contradictory assertions have been made. The elements in question are composed of a fluid substance readily obedient to the action of external forces. ”

“ The photographic plates, moreover, which are prepared now, are not only sensitive to all the elementary rays which excite the retina, but the power also extends into those ultra-violet regions and the opposite regions of dark heat in which the eye has no power. ”