OF MICROSCOPIC OBJECTS. 197 
taken that the tape be not too tight, else the circulation will 
be very slow or altogether stopped. The wood must now be 
fixed upon the stage, with the aperture under the object- 
glass: this is sometimes done by simply binding it, or a 
spring is fixed so as to accomplish the same object without 
so much trouble. With a half-inch power the blood may 
be seen to flow very distinctly. The frog may be used for 
hours if care be taken to prevent the web from becoming 
dry, by wetting it with a little water from time to time. 
The piece of wood or cork upon which the frog is laid is 
often made to give place to the “ frog-plates,” supplied by 
opticians. These are made of brass, somewhat resembling 
the piece of wood above recommended, but each maker’s 
pattern differs, according to his own taste. 
The tongue of the frog is also sometimes used for the 
purpose of showing the circulation of the blood, which is 
done in the following manner :—The body is wrapped with 
calico, and made fast to the plate as before, only the mouth 
of the frog is brought to the opening. The tongue is then 
gently drawn out of the mouth and pinned down over the 
aperture, when the circulation will be well shown. But, as 
Dr. Carpenter observes, the cruelty of this mode of treat- 
ment is so repulsive that it is unjustifiable. 
Tadpoles of the frog (which, of course, are only obtainable 
in their season) are good subjects for showing the circula- 
tion of the blood. They are best suited for the microscope 
when about one inch long. The tadpoles of the newt and 
toad also are equally suitable. They may be placed in a 
very shallow glass trough with a little water, and a narrow 
band of linen bound lightly round in some part not required 
for examination, to keep them from moving; or they may 
be laid upon a glass plate with a drop or two of water, and 
a thin glass covering lightly bound upon it. Dr. Carpenter, 
however, places them first in ccld water, gradually adding 
warm until the whole becomes about 100°, when the tadpole 
becomes rigid, whilst the circulation is still maintained. I 
have not, however, found this necessary, the thin glass ac- 
