First, he investigated the absorption of the chemical rays by transmission through different media; through solids (transparent, of course), through liquids, and through gases and vapours; the only alteration he made in his general mode of experimentation being 90 100 110 120 130 140TM 150 1.60 170 180 ICO BD FGH Ptin O PtinN Pt in Cl Pt in Co FIG. 32*.-Copies of Dr. Miller's maps of the ultra-violet spectrum of platinum in that in the case of the absorption of gases and vapours * These maps have been obligingly placed at my disposal by Messrs. Long- light which passed to the telescope was compelled to traverse a thickness of vapour according to the length of the tube employed. In that way he not only determined the absorption of equal lengths of different vapours amongst themselves, but the absorption of different lengths of the same vapour; his paper is thus one of the most important contributions to spectroscopic knowledge that I am acquainted with, and I hold that the chief importance of it is the application of photography to spectroscopic observations. There are few observations more difficult, I think, than spectroscopic ones, while from the little experience I have had, I should think there is nothing more easy than to produce passable photographs of spectra. $5 M. Mascart's Maps. That, then, was in the year 1862. In the year 1863 we have another equally distinct advance to chronicle, but this time the work is done in France. M. Mascart, a name very well known to physicists-undertook a tremendous work, which he has not yet completed, namely, a complete investigation of the ultra violet solar spectrum.* Instead of using a quartz prism, as Dr. Miller had done before him, M. Mascart uses a diffraction grating, that is to say an instrument by means of which the light is not refracted, as in the case of the prism, but diffracted by an effect of interference of * 'Annales scientifiques de l'Ecole normale Supérieure." Vol. for 1864, p. 219. fine lines ruled on glass. M. Mascart has shown it to be possible, by reflecting light from the first surface of the diffraction gratings, to get light diffracted without its going through glass at all. In this way, therefore, we avoid altogether the imperfect transparency of glass. Prof. Mascart has gone on advancing every year, until now he has completed, almost entirely by means of photography, a map of the solar spectrum extending about as far as the line called T. There he finds the solar spectrum ends; but in the case of a great many vapours, such, for instance, as that of cadmium and others of like nature, he finds he can go on photographing very much further, and he has been able to photograph to a distance, five or six, or even seven times as far from the line H as H is from A. § 6. Rutherfurd's Photograph of the Solar Spectrum. A very beautiful reflex action of spectroscopy on photography may now be referred to for a moment. Rutherfurd, whose name is associated with that of Delarue in the matter of celestial photography, was not pleased with the action of reflecting telescopes. He lives in New York, and I suppose New York is as bad as London for tarnishing everything that the smoke and atmosphere can get at; and he came to the conclusion that he must abstain from celestial photography altogether, or else make a lens-and a lens with Mr. Rutherfurd means something over 12 in. diameter-which should give him as perfect an image in New York with 15in. of glass, as a perfect reflector of equal aperture. Mr. Rutherfurd, who never minces matters, knowing that it was absolutely impossible to get such a lens as this from an optician, who of course neglects almost entirely the violet rays-the very rays which he wanted -in constructing an ordinary telescope, determined to make such an one himself. Thinking about the matter, he came to the conclusion that in any attempt to correct a lens of this magnitude for the chemical rays, the use of the spectroscope would be invaluable. He therefore had a large spectroscope constructed, in order to make a large telescope, and as a result of this we have as distinct an improvement upon the instruments which we owe to the skill of those who first adopted the suggestion of Sir John Herschel and brought together the chemical and the visual rays, as the improvement we owe to Herschel was upon the instruments which dealt simply with the visual rays. Mr. Rutherfurd simply discards the visual rays, and brings together the violet ones; the result of his work being a telescope through which it is impossible to see anything, but through which the minutest star, down, I believe, to the tenth magnitude, can be photographed with the most perfect sharpness. This is the instrument of the future, so far as stellar astronomy is concerned. Having thus achieved what he wished in the construction of this instrument, and having the spectroscope, Mr. Rutherfurd commenced a research which, I am sorry to say, he has never published, for it would be of the greatest value, upon the best kinds of collodion and the best arrangement of lenses for spectrum photography. He found that some collodions are so local in their action as to be almost useless for that reason, and that others are so general in their action that they are also almost useless for the exactly opposite reason. Mr. Rutherfurd's contribution to photographic spectroscopy, his photograph of the solar spectrum from F to H, is quite as admirable and excellent as his photograph of the moon the photograph is a refraction photograph, that is to say prisms were used, and, more than this, the prisms were of glass. It therefore, extends only a very little distance beyond the H lines. But America was not satisfied with this, and in the person of Dr. Draper, the son of the Professor Draper whose name is so honourably associated with the commencement of work done in photography thirty years ago, has just now photographed the solar spectrum far beyond H with a Rutherfurd diffraction grating. 7. The Use of the Compound Slit. We have already seen how exceedingly important it was to use a slit instead of a round hole in these experiments. It was shown by Wollaston with regard to eye observation, and by Becquerel and Draper with regard to spectrum photography. |