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 g. little water from time to time. 

 The piece of wood or cork upon which the frog is laid ia 

 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 moutli 

 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 obtainabl i 

 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 cold water, gradually adding 

 warm until the whole becomes about 100, when the tadpole 

 becomes rigid, whilst the circulation is still maintained. I 

 vbave not, however, found this necessary, the thin glass ao- 



