128 



TESTING THE MICROSCOPE. 



nature of the details is complete, the only difference being in 

 magnitude is added, in the second place, the important fact that 



their production is very simple and 

 inexpensive. Air-bubbles of every 

 range of size are very easily ob- 

 tained. By dissolving gum-arabic in 

 water they are formed abundantly, 

 because the air contained in the 

 fragments of gum is not completely 

 absorbed. They are also readily 

 obtained by placing a drop of glyce- 

 rine, or oil, upon a plane glass slide 

 and beating it with a knife-blade 

 until it begins to froth; it should 

 then be protected by a cover-glass, 

 care being taken to place at the 

 edges some suitable substance 

 (threads of cotton, slips of paper, 

 &c.) to prevent undue compression. 

 Minute bubbles of carbonic acid 

 gas serve the same purpose, and are 

 easily obtained by pouring a drop of dilute hydrochloric acid upon 

 powdered chalk. With a large selection of air-bubbles of different 

 size it is possible, with the same wire-gauze, by merely changing 

 the object-distance slightly, to obtain images that reach the limit 

 of discrimination for all powers. 



In order to determine with certainty the size of the network at> 

 the limit of discrimination, we advise as 

 follows : A square piece of wire net, con- 

 taining about 5x5 to 10x10 meshes, is 

 enclosed by strips of black paper (Fig. 81), 

 and is then placed between the mirror and 

 the air-bubble in such a position that its 

 image is just perceptible. If one side of the 

 square is measured, it is only necessary to 

 divide by the number of interstices con- 

 tained in this side, to determine the distance 

 of the interspaces that is, the diameter of the meshes. For 

 instance, if the entire square has a diameter of 7*5 mic., as repre- 

 sented in the Fig., the distance of the wires (disregarding on both 



FIG. 80. 



FIG. 81. 



