On the Employment of Coloured Glass in Microscopy. 103 
to spread out again. Glycerine was tried for the mounting medium, 
but the appearances were more natural in the acetate potash 
medium. More than a hundred specimens were treated in various 
ways, too long to enumerate here, the preference being given to the 
plans fully detailed. 
HI. — On the Employment of Coloured Glass in Microscopy. 
By M. Mouchet. 
I believe that the use of coloured glasses in microscopical studies 
may facilitate the examination of many objects which would other- 
wise remain drowned in a flood of light ; I believe also that the use 
of them must remarkably spare the eyesight. It is many years 
since Ch. Chevalier first employed tinted glasses, which he placed 
above his lenses in order to temper the intensity of the light. By 
the use of those glasses, or of silvered glass, sunlight may he made 
use of, and may thus replace all the other systems of illumination 
which are intended to supply the brightness that powerful objec- 
tives tend to destroy. Now, what position should these different 
glasses occupy ? We can count four : before the mirror ; under the 
object ; between the objective and the eye-piece ; finally, above the 
eye-piece itself. To place them before the mirror necessitates 
the employment of glasses of a certain size ; to place them under 
the object increases the thickness of the plate and prevents a very 
oblique light from passing through ; to fix them in the body of the 
instrument causes trouble and a loss of time, for the microscope 
must be taken asunder. Above the eye-piece appears to me the 
most suitable place. For this purpose, I have constructed an 
apparatus, consisting of a kind of revolving diaphragm pierced by 
nine openings, each one centimetre in diameter, and placed exactly 
opposite the eye-piece. Seven of these openings are fitted with 
coloured glasses corresponding to the colours of the solar spectrum ; 
another with a silvered glass, according to Foucault’s process ; and 
the ninth with a very finely roughed glass, which may be removed 
at will, and thus leave one opening free. This revolving diaphragm 
is attached to a ring placed on the tube which receives the eye-piece, 
and is fixed to the tube by means of a vis de serrage, as in certain 
“ chamhres claires ” (cameraelucidae ?). The apparatus has some 
analogy to that which Mr. Collins calls “ light corrector,” and 
which he places on the stage of the microscope. 
It is evident that when coloured glass, as, for example, cobalt 
blue glass, is used, we have not a pure monochromatic light; 
but this glass replaces, if not with advantage, at least without in- 
