54 
The Presidenfs Address. 
adapted for the study of coloured solutions in test-tubes, but 
cannot be employed with objects less than -ro^^ of an inch in 
diameter. 
For this reason it is more generally advantageous to 
employ a form of apparatus with the slit in the focus of the 
upper lens of the eye-piece, made achromatic ; and it is also 
very much better to have such arrangements that two spectra 
can be compared side by side, as described in his paper in 
the January number of the 'Popular Science Review,^ 1866, 
p. 66. 
With the Sorby-Browning spectroscope, the spectra of very 
minute bodies can be seen to great advantage either by trans- 
mitted or reflected light ; and their being only partially trans- 
parent and of considerable thickness does not signify much. 
'Of course, the use of such an instrument is almost entirely 
restricted to coloured bodies, though in some cases the colour 
may be very faint ; but whenever colour is an important 
character, it appears that such a method of investigation 
should not be neglected. The value of the results depends 
very much on whether the spectra do or do not give well- 
marked absorption bands ; and there are many cases in which 
the facts are unfortunately very indefinite. 
As far as can be judged at present, the chief use of the 
instrument will be in forming a more definite opinion respect- 
ing the nature of solutions, coloured either naturally or by 
the addition of tests ; to the study of blowpipe beads, and of 
natural and artificial crystals; and, in some cases, to the 
determination of the nature of the minerals met with in the 
sections of rocks and meteorites in which chemical analysis 
cannot be employed. 
There are also some branches of natural history and phy- 
siology to which it might be usefully applied ; but hitherto 
Mr. Sorby has been more anxious to bring the instrument 
itself to perfection, and to establish its fundamental prin- 
ciples, than to employ it extensively in deciding any other 
practical question than the detection of minute blood- stains 
in cases where ordinary microscopical examination could not 
yield decided results. As I have previously stated, Mr. 
Huggins and Mr. Wenham adopted an opposite course to 
Mr. Sorby : instead of applying the microscope to the spec- 
troscope, which was Mr. Sorby^s plan, they applied the 
spectroscope by using it as an eye-piece to the microscope ; 
and, in addition to the examination of solutions or trans- 
parent objects, they were enabled for the first time to inves- 
tigate the spectra afforded by small and strongly-illuminated 
objects that were opaque. 
