Microscope Spectrum Apparatus. By H. C. Sorby. 199 
other at the side of the eye-piece. The two spectra can also be readily 
made equally bright, which is of the very greatest importance 
where the instrument is used for comparative quantitative analyses.* 
The illumination of the object placed in front of the small reflecting 
prism can easily be regulated by means of a large bull’s-eye con- 
denser, and that of the object on the stage by using a plano-convex 
condensing lens of about • 6 inch diameter and of about the same 
focal length placed below the stage. With the ordinary diaphragm 
having several holes of various sizes, or with an iris diaphragm, 
on reducing the amount of light we also limit the width of the 
spectrum. In order to avoid this it is requisite to reduce the light 
by using a narrow but not shorter beam, and for this purpose I 
have found a diaphragm with a volute opening extremely convenient, 
since by turning it round the amount of light can be regulated to 
a great nicety without in any way limiting the length of the 
spectrum. 
2. Measurement of Bands. 
In many of my former publications I have described the manner 
in which I have used as a scale of measurement the dark bands in 
the spectrum of the light passing through a plate of quartz between 
two Nicol’s prisms, the thickness of the plate being such that 
the whole visible spectrum contains twelve dark hands, and the 
Fraunhofer’s line D is half-way between the bands 3 and 4, 
counting from the red end. With the binocular instrument the 
hands seen in any spectrum could he measured by means of an 
ordinary ruled micrometer in the eye-piece, but it is not only very 
difficult to see lines over the blue, hut the least movement of tbe 
apparatus would throw all out of adjustment. Mr. Browning’s 
method of measurement, described in this Journal (vol. iii., p. t>8), 
though extremely useful for certain purposes, cannot be conveniently 
adapted to the binocular instrument, and even when employed in 
the manner proposed by him, has unfortunately several serious 
defects for expeditious practical working. Slight movements 
throw it out of adjustment, and it is very inconvenient to 
have to measure in all cases from the sodium line, and to have 
to read off from the small graduated circle ; but a still greater 
objection is that, if the bright dot be arranged over the centre 
of an absorption band when the slit is narrow, it is no longer 
over the centre when the slit is made wider, and such an alteration 
in width is often very desirable. For these reasons I make use of 
Mr. Browning’s plan only for particular purposes, and for general 
work still employ the interference scale, since no movement of the 
instrument can alter the relative position of the bands in the scale 
* For illustrations of such analyses I refer to ray paper in the 1 Proceedings 
of the Royal Society,’ 1873, vol. xxi., p. 442. 
VOL. XIII. 
Q 
