112 BiiowNiNG^ on the Spectroscope and Microscope. 
tained. The particular portion of the object of which it is 
desired to examine — the spectrum — is then brought to coin- 
cide with the fixed edge of the slit, and the movable edge is 
screwed up, until a brilliant-coloured spectrum is produced. 
The absorption-bands, if the specimen gives any, will then 
be readily found by slightly altering, the focus of the micro- 
scope. 
This contrivance answers perfectly for opaque objects, 
without any preparation ; it is not expensive, and it does not 
add appreciably to the bulk of the^microscope. "When desira- 
ble, the same prism (Fig. 6) can be placed below the stage, 
and a micrometer used in the eye-piece of the microscope, 
thus avoiding multiplication of apparatus or increasing the 
expense. By Mr. Sorby^s kindness I exhibit an almost 
microscopic spot of blood on a card, the absorption-bands in 
which can be readily seen with this arrangement. On the 
table I have also the pleasure of exhibiting some very beauti- 
ful crystals and solutions of Mr. Sorby^s preparation. 
It has been urged that there is a great similarity between 
the absorption-bands given by various substances. To this I 
answer that I have never seen two spectra alike ; and I beg 
to direct your attention to the various diagrams which I ex- 
hibit, the best of which are Mr. Sorby^s, in proof of the cor- 
rectness of my assertion. 
In the ^ Chemical News^ of June 2nd, Mons. Marc Dela- 
fontaine has proposed to make use of the absorption-bands 
to distinguish between the salts of erbium, terbium, and 
didymium. 
Mr. Sorby says of the correct performance of a spectrum 
adaptation, " the best tests are — first, that the absorption- 
bands in blood can be seen when they are very faint ; second, 
to well divide the bands in permanganate of potash; and, 
third, to see distinctly the very fine line given in the red by 
a solution of chloride of cobalt. In a concentrated solution 
of chloride of calcium there is a line so fine that it looks like 
a Fraiinhofer^s line. An instrument that shows all these 
well is all that can be desired.'^ I am glad to say that Mr. 
Sorby, to whom we are so much indebted for the original 
idea, is actively pursuing his investigations in a new direc- 
tion in this very interesting branch of science. 
It is very desirable that others should also give their atten- 
tion to this matter, each choosing different subjects for inves- 
tigation. The recent important discoveries of Dr. Bence 
Jones, F.Pt.S., and Dr. Dupre, on the detection of lithium in 
the eyeball a few hours after its administration, could be ad- 
vantageously carried further in this manner ; and the success 
