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FEBRUARY 27, 1880.*] THE PHOTOGRAPHIC NEWS. 99 approved, is at Kilburn, where a considerable staff of em ployes are engaged. “ Secrets ? Lord bless you! I have none, I may reply with Addison’s knife grinder,” was Mr. Bassano’s remark, when we asked but for a general view of bis establishment. “ I have met with some success, but the only secret which has tended to it has been that I have brought to bear upon my work whatever art cultivation, inclination and circum stances have fostered.” We have only to assure our readers that what we saw at the Old Bond Street Gallery practically confirms these words. The “ At Home ” next week will be at Messrs. Wil liams and Mayland’s Studio, Regent Street. ON THE PHOTOGRAPHIC SPECTRA OF STARS. BY WILLIAM HUGGINS, D.C.L., LL.D., F.R.S.* A SMALL part only of the radiations of a luminous body comes within the power of the eye. There is a large range of shorter vibrations beyond the violet, and also of larger vibrations below the last visible part of the red, which cannot affect us as light. Your distinguished Vice-Presi dent, by the discovery of a new molecular condition of s Iver bromide, has brought these last—the infra-red equally with the former, the ultra-violet rays—within the power of the photographic plate. The new work, of which I am about to speak, has to do with the more refrangible, the violet, end of the spectrum. The eye observations of the spectra of stars, which I made some years ago, included the portion of the visible spectrum from about 0 in the red to a little beyond G in the blue. The photographic spectra begin a little below G, and carry our knowledge beyond 0 to about S in the ultra-violet. In endeavouring to obtain photographs of the spectra of stars, two serious difficulties present themselves. One of these arises from the great feebleness of the light of a star after dispersion by a prism, the other from the apparent motion of the star in consequence of the earth’s motion of rotation. It was necessary, therefore, to devise an apparatus which should produce on the plate a well defined spectrum, full of fine details, with the least possible loss of light. As glass is but imperfectly transparent to light beyond the visible spectrum, it was necessary to avoid the use of this substance. The telescope was a reflector of the Cassegrain form, having a metallic speculum eighteen inches diameter. The form of spectrum apparatus is represented in the ac companying wood-cut:— Fig. a is a base plate with bevelled edges, which slides with a suitably grooved plate fixed at the end of a telescope. b is the slit, having a width of 3}oth part of an inch ; c is the prism of Iceland spar, a substance very transparent to the ultra-violet rays, and possessing a power of dispersion equal to moderately dense flint. The lenses, d and e, are of quartz. The plate is placed at F, and inclined so as to bring as large a part as possible of the spectrum to focus upon it. The photographic spectra taken with this appa ratus measure half an inch from G to D, and the definition is so excellent, that seven lines may be seen between II and K in the solar spectrum. The other difficulty, arising from the star’s apparent motion, required a special arrangement to enable the star’s image to be brought upon and kept accurately within the very narrow chink, the 3both of an inch wide, through which the light must pass. In addition to a massive equatorial mounting, and a driving clock of great excellence, due to the inventive skill of Mr. Howard Grubb, the arrangement shown in fig. 2 was adopted. Fig. 2 shows parts of the telescope. The spectrum appa ratus a is fixed so that the slit may be exactly at the prin- * Read before the Photographic Society of Great Britain, 1. cipal focus of the mirror b. Over the slit is placed a polished silver plate, c, with an opening corresponding to Fig. 2. the slit. By means of a small mirror, d, artificial yellow light is thrown upon this plate, Behind the hole in the
