40 VIEWING MICROSCOPIC OBJECTS. 



the scales from the podura springtail, discovered by Mr. T. Carpenter, 

 and on which the scales are only just discovered by the most perfect 

 instruments. When the penetrating power is thus ascertained, its 

 defining power may be determined, by inspecting a leaf of the moss 

 of a species of the genus hypnum, which requires a considerable 

 penetrating as well as defining power fully to develop the hexagons 

 which constitute its fabric, making out a luminous nucleus to each 

 which should be sharply defined ; and of the same shape with the 

 outer hexagon. As opaque test objects, the bat's hair shown in figs. 

 26 and 27, and the mouse hair, figs. 28 and 29, may be considered 

 excellent tests, when the markings and outlines are well defined. 

 These objects maybe also examined with transmitted light with the 

 same advantages. 



Fig. 26. Fig. 27. Fig. 28. Fig. 29. 



28. The white letters on a black ground, seen on a piece of 

 enamelled watch plate, is perhaps one of the best tests to determine 

 the quantity of chromatic or spherical aberration in a lens ; indeed, to 

 detect the latter error, an artificial star is perhaps the best thing, as it 

 requires considerable defining power to show well. The artificial star 

 may be made by taking a very small globule of pure mercury kept in 

 gum-water, and securing it to a black ground, as burnt cork or black 

 paper, or a globule of platina fused by electricity, and attached to a 

 black ground. 



29. In examining these test objects, the direction and quantity of 

 light must be carefully attended to, nor must it be injured or mutilated 

 by the reflector, condensing lens, or other diaphonous body through 

 which it may pass to the object. When an instrument can show these 

 proof objects, it may with certainty be pronounced effective. It 



