Circulation of the Blood. 



141 



When making the drawing from which Plate VIII., 

 fig. 2, is taken, I often put the fish back into the 

 water, choosing for the purpose the interval when I 

 required to go over my sketch with pen-and-ink. 

 After a while, I placed the fish again on the slide, 

 sketched more, and ascertained points that had been 

 doubtful in the inking. There was no difficulty about 





/ 



A It 



No. 24 Arrangements for exhibiting the circulation. 



finding the portion wanted on its tail-fin. The 

 "achromatic condenser/' which I always use when 

 viewing the circulation, makes a disc of light exactly 

 where the object is placed ; so that with the unas- 

 sisted eye I could judge when I had placed the right 

 part of the fin over the light, and some familiar por- 



