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II. — On New and Improved Microscope Spectrum Apparatus, 
and on its Application to various branches of Research. 
By H. C. Sorby, F.B.S., &c., Pres. K.M.S. 
( Bead before the Royai. Microscopical Society, April 7, 1875.) 
1 . Description of Apparatus. 
The general construction of the spectrum eye-piece which I carried 
out some years ago in conjunction with Mr. Browning, is so well 
known that I will not occupy time in describing it, but^confine 
myself to what is new. 
Having lately felt the need of a complete yet. compact instru- 
ment to take with me from home, I was anxious to arrange so that 
I could use it as a binocular microscope, and at once examine the 
spectrum of any object by merely inserting a slit into one of the 
eye-pieces, like a micrometer, and then placing the compound direct 
prisms over the ordinary eye lens. As usually made, this does not 
allow convenient space for focal adjustment of the spectrum, and is of 
too short focal length for good definition. Moreover, being a single 
simple plano-convex lens, the spherical aberration is objectionable ; 
but by inserting a plano-concave lens of 7 inches negative focus under 
the prisms, the definition is all that could be desired, and the focal 
length is at the same time increased to what is practically the most 
convenient. In order to be able to compare together two spectra 
side by side, a small reflecting prism is fixed over half the slit, with 
another slit in front of it, which can be made narrower or wider 
so as to regulate the amount of illumination ; since otherwise the 
spectrum of the light merely reflected by the prism would be much 
too bright for comparison with that of the magnified object on the 
stage. A small side stage to hold objects for comparison is per- 
manently fixed on the main body of the instrument, in such a way 
as not to interfere with its use as a simple binocular microscope. 
All these arrangements have turned out very satisfactory, being 
not only compact and convenient, but also as good in all other 
respects as is compatible with the eye-piece spectrum method. I, 
however, have found that for the study of the spectrum of solutions 
in small glass cells or test tubes, or that of any objects of sufficient 
size like portions of leaves or feathers, or in fact of anything not 
less than x Vth of an inch in diameter, it is far more convenient to 
use the binocular spectrum apparatus described in my paper in the 
‘ Proceedings of the Boyal Society,’* since we then view the two 
spectra with both eyes under more similar conditions, and can use 
two of the small glass cells at the same time, and illuminate both with 
one lamp with far greater ease than when one is on the stage and the 
* 1867, vol. xv., p. 443. 
