THE SPECTRUM-MICROSCOPE. 
69 
object on the stage may be first examined by means of an or- 
dinary eye -piece^ and placed in the centre of the field; and 
then this eye-piece substituted and the spectrum examined 
and compared with the natural spectrum due to the light 
passing through the opening' at /, or with that of any coloured 
substance placed in front of that opening on the stage h i. 
It is; however; far better to use a binocular microscope; since 
then one tube can be used to examine the object; and place it 
in the centre of the field; and the eye-piece and prisms I have 
described can be kept permanently in the other tube to study 
its spectrum. By pulling out the other eye-piece; the focal 
length of both can easily be made the same. Of course; a high 
power could be used; but it is better to use as low as conve- 
nient; in order to obtain more light. If the object be too 
small to cover the whole length of the slit; I have a small 
slide worked wdth a screw to reduce its length to the size 
of the object. 
I always feel that the detailed description of apparatus is 
tedious and unsatisfactory. More could be learned in a few 
minutes* inspection than by reading pages of description;, 
which often make what is very simple appear very compli- 
cated. Still; some description of the apparatus was essential; 
and I trust what I have said may suffice to explain its general 
construction; without trenching on space that should be devoted 
to an account of the objects to which it may be applied. 
Summing up; then; the advantages of this form of appara- 
tus; I may say that it enables us to examine the object itself; 
and its spectrum; without any changes of focus or position. 
By simply removing the prisms; we can easily see whether any 
very minute object is in front of the slit; and; if need be, 
open the slit wider so as to be sure that it is in the best 
position ; and we can compare its spectrum; — and the whole 
of the rest of the field of view is quite dark; — side by side 
with that of white light; or of any other larger object placed 
on the stage at the side of the eye-piece ; and all the spectrum 
is in focus at the same time. 
In studying the spectra of coloured solids or liquids; it is 
very essential to distinguish between what may be called 
characteristic and uncharacteristic spectra. Unfortunately; 
very many substances belong to the latter class; and it is 
this, more than anything, which limits the application of this 
method of research. In the case of these uncharacteristic 
spectra, when the object is only slightly coloured on account 
of being very thin, or a very dilute solution, it begins to 
absorb either the red or the blue end of the spectrum ; and 
on increasing the thickness or the strength of the solution., 
