146 Royal Society : — Mr. H. C. Sorby on Analysis of Animal 



by turning it sideways ; and thus we can save much time, and 

 quickly ascertain what strength of solution would give the best 

 result. Very frequently we obtain an excellent spectrum in one di- 

 rection with one reagent, and in the other with another, without 

 further trouble. I have constructed a small stage, too complicated 

 to describe in writing, which enables me at once to examine solutions 

 in two such tubes, either endways or sideways, and compare their 

 spectra side by side, or to use test-tubes, or to fix the small appa- 

 ratus which I have contrived for accurately measuring the spectra. 

 This is of such great importance in these inquiries that I must de- 

 scribe it in some detail. 



3. Scale of Measurement. 

 It consists of two small Nicol's prisms, and an intermediate plate 

 of quartz. If white light, passing through two such prisms, with- 

 out the plate of quartz, be examined with the spectrum-microscope, 

 it of course gives an ordinary continuous spectrum ; but if we place 

 between the prisms a thick plate of quartz or selenite, with its axis 

 at 45° to the plane of polarization, though no difference can be seen 

 in the light with the naked eye, the spectrum is entirely changed. 

 The light is still white, but it is made up of alternate black and co- 

 loured bands, evenly distributed over the whole spectrum. The num- 

 ber of these depends on the thickness of the depolarizing plate, so 

 that we may have, if we please, almost innumerable fine black lines, 

 or fewer, broader bands, black in the centre and shaded off at each 

 side. These facts are of course easily explained by the interference 

 of waves. It would, I think, be impossible to have a more conve- 

 nient or suitable scale for measuring the spectra of coloured solids 

 and liquids. If we use a micrometer in the eyepiece, an alteration 

 in the width of the slit modifies the readings, and the least move- 

 ment of the apparatus may lead to error, whereas this scale is not 

 open to either objection. Besides this, the unequal dispersion of the 

 spectrum makes the blue end too broad, so that a given width, as 

 measured with a micrometer in the eyepiece, is not of the same op- 

 tical value as the same width in the red. The divisions in the in- 

 terference-spectrum bear, on the contrary, the same relation to the 

 length of the waves of light in all parts of the spectrum, and no 

 want of adjustment in the instrument alters their position. As will 

 be seen from the drawing (fig. 1), the unequal dispersion makes the 

 distance between the bands in the blue about twice as great as in 

 the red. The perfection of a spectrum would be one in which they 

 were all at equal intervals ; but possibly no such uniform dispersion 

 could be produced. By having a direct-vision prism, composed of 

 one of flint-glass of 60°, and two of crown-glass of suitable angle, 

 we can place it over the eyepiece, and may diminish the dispersion 

 at the blue end, or increase that at the red end, by turning it in one 

 position or the other, and thus see either end to the greatest advan- 

 tage. It is, of course, very easy to draw spectra on this principle, and 

 give all parts equal prominence, and not an unduly compressed red, 

 and an unduly expanded blue end. Thus drawn, the spectra are far 

 more uniform in many of their characters, and some general laws 



