EXPERIMENTS IN MOUNTING THE AMPHIPLEURA PELLUCIDA. 133 
In conclusion, I wish to impress upon microscopical workers to 
find out as near as possible the refractive index of the material 
they are working on, because if the material they are working 
with has the same refractive index as the medium they are going 
to mount in, practically speaking nothing can be seen less it is 
a stained preparation. 
My usual plan is to put a drop of the medium on a glass slip; then 
a prepared cover-glass is laced in position and examine under 
the ees ; I then immediately see whether I have a mounting 
medium o igher refractive index than the silica of the diatom, 
which is 1 43 if the diatoms are visible and well defined I go on 
the contents of these cell-walls escape, and when the preparation 
is mounted an abnormal structure is sure to be seen. I contend 
that the contraction and eo of any tissue in preparation is 
abnormal, and will give a false earance to the object under 
must be enormous. I hav ye that if I allow it toactona 
pus cell . more than one sainuto the cell will burst, laren the 
ological— 
excuse me for travelling beyond my ped 
