THE SPECTRUM-MICROSCOPE. 
67 
When I first commenced this subject, I confined myself to 
such a form of apparatus as could be easily constructed with an 
ordinary simple triangular prism, which was placed below 
the achromatic condenser. I described the details of the 
arrangements, and a number of facts which could be learned 
by means of the instrument, in a paper in the Quarterly Journal 
of Science , April 1865, vol. ii., 198. However, I afterwards 
perceived that if a suitable direct-vision prism could be con- 
trived, to use over the upper lens of the eye-piece, it would 
present many great advantages. Mr. Browning lent me a 
number of different prisms, made others at my suggestion, 
and has carried out my views with such zeal, that we have at 
length succeeded in obtaining what appears to me a perfectly 
satisfactory form of instrument. I shall not attempt to de- 
scribe all the minute points in its construction, since that is 
a question which more concerns an instrument-maker than a 
general reader or experimenter, but will give such a general 
description as may serve to explain its capabilities. 
The prisms employed to obtain the spectrum are composed 
of two rectangular prisms of flint glass, between which is a rect- 
angular prism of crown glass, and at each end another prism 
of crown with an angle of about 75°, as shown in section by 
fig. 2. These are all cemented together with Canada balsam ; 
and since we get the sum of the dispersive action of the 
two flint prisms corrected for refraction by the three of crown 
glass, we obtain a very satisfactory spectrum with perfectly 
direct vision. In some cases a less dispersion is desirable, 
and I therefore have another compound prism composed of 
one rectangular flint and two crown glass prisms of about 
60 ° ; and in a few cases, when a wider dispersion is required, 
I combine the two series. On the whole, however, the com- 
pound prism shown in the fig. is just of that medium power 
which is required in by far the larger number of cases. These 
analyzing prisms are mounted in a tube having a cap with an 
elongated opening at u, a circular stop at b, and a free part of 
tube b c, which fits over the upper lens of the eye-piece like 
an ordinary cap. 
Since some persons who have not paid attention to the 
ordinary kind of spectroscope, may be induced to take an 
interest in its application to the microscope, it may, perhaps, be 
well to say a word or two with reference too the use and need of 
a slit. Without it, on placing the prism over the eye-piece, all 
the objects in the field would appear fringed with the prismatic 
colours, without anything definite being seen. What is wanted 
is a narrow beam of light, which, on passing through the prism, is 
spread out in such a manner, that, if any particular rays are defi- 
cient in the light, the fact may be made apparent by there being 
