224 THE AMERICAN MONTHLY [Aug 
the room darkened as much as possible, when it will be 
found, if the prisms are nicely adjusted, that a brilliant 
image of the spectrum is projected. By slightly moving 
the prisms or the microscope, the exact wave length of 
light that is desired for the work in hand can be selected, 
and that wave length used for the examination or photo- 
graphy of objects. The brilliance of illumination secured 
under these circumstances is so great that light well into 
the violet end of the spectrum can be satisfactorily 
utilized, and the cere? power of objectives increased 
thereby. 
No ordinary light filter passes light of one wave length 
or even approximately so; by means of a heliostat a far 
more exact result can be’secured, especially when by the 
method named above, a spectrum measuring 20 inches or 
more in length can be obtained at the microscope mirror, 
Those who worked with a heliostat once will anticipate 
further sunny hours for improving their acquaintance - 
with it. 
LABELLING SPECIMENS. —It is well in mounting objects 
to have some ready means of marking them for future 
identification. The so-called grease pencil for writing 
on glass, which may be had in various colors, and can be 
purchased from many opticians, is one of the handiest 
and neatest means, especially as the writing can be re- 
moved subsequently with a little warm water. 
Another device where labels are not at hand is to pass 
the glass slip across the tongue, allow the dampness to. 
dry, after which an ordinary pen and ink can be used for 
writing upon the portion so treated. 
In the case of specimens mounted on cover glasses or 
slides, which it is required to identify after passing 
through various solutions, the following method is recom- 
mended :— / 
Mix into equal parts of egg, albumen and glycerine, 
