sl^tTK'] structure of Diatoms, &c. 159 
the hght which enters direct into the vertical face of the prism, 
and is reflected upwards through the under-side by means of the 
mirror /. The diagonal face of the prism is surmounted by a 
C 
dark box, through which is introduced the microscope body, with 
object-glass, g. The side of this box has a sliding shutter opening 
level with the face of the prism, for the introduction of slides and 
detached objects. 
The two lights simultaneously transmitted through the rectan- 
gular faces of the prism will both be totally reflected from the 
diagonal surface, and if this is clean not a trace of light will be 
seen in the dark chamber. A fragment of thin white paper laid 
on the prism will be quite invisible under the microscope. If now, 
with a needle-point, we touch the paper with a minute drop of 
water, so as to bring a spot in aqueous contact with the prism, 
this will destroy total reflexion at the part, which will appear 
brilliantly luminous. 
If a water-snail, aquatic larva, or caterpillar is placed on the 
prism with a drop of water, the whole of the interior anatomy is 
distinctly seen, as the light enters the part only of the object in 
contact with the glass, and is diffused throughout the body. When 
objects mounted on ordinary microscope slides are to be viewed, 
some water must be placed on the back of the prism ; and if the 
object is a moving one, it may be laid on a blank slide, and this 
traversed over the prism Vv^ith water between. 
Though this arrangement displays large transparent objects, 
