optics. 281 



brass ring with the point of a penknife : the mica 

 will then fall out, if you but turn the sliders ; and 

 after putting what you please between them, by 

 replacing the brass rings, you will fasten them as 

 they were before. It is proper to have some sliders 

 furnished, but without any objects, to be always in 

 readiness for the examination of fluids, salts, sands, 

 powders, the farina of flowers, or any other ob- 

 jects of such sort as need only be applied to the out- 

 side of the mica. 



The circulation of the blood may easiest be seen 

 in the tails and fins of fishes, in the fine mem- 

 branes between a frog's toes, or, best of all, in the 

 tail of a water-newt. If your object be a small 

 fish, place it within the tube, and spread its tail, or 

 fin, against the side thereof ; if a frog, choose such 

 a one as can but just be got into your tube, and 

 with a pen, or stick, expand the transparent mem- 

 brane between the toes of the hind foot as wide as 

 you are able. When your object is so adjusted, 

 that no part thereof can intercept the light from 

 the place you intend to view, unscrew the long 

 screw C, and thrust the tube into the arched 

 cavity, quite through the body of the microscope ; 

 then screw it to the true focal distance, and you 

 will see the blood passing along its vessels with a 

 rapid motion. 



Make use of the third or fourth magnifier for 

 frogs or fishes ; but for the tails of water-newts, the 

 fifth or sixth will do, because the globules of their 

 blood are twice as large as those of frogs or fish. 

 The first or second magnifier cannot well be em- 

 ployed to this purpose ; for the thickness of the 

 tube wherein the object lies will scarce admit its 

 being brought so near as the focal distance of the 

 magnifier. 



