TEST OBJECTS. 469 



The figures represented by B and D were drawn by a mag- 

 nifying power of 500 diameters ; but as a test of the defining 

 power of a half-inch object-glass they should be chiefly 

 employed. When viewed as an opaque object, this hair is 

 very beautiful, the dark parts will then appear very much 

 more light than those that are transparent, and the structure 

 wiU be imagined to be quite the opposite of that seen by 

 transmitted light. 



Hair of the Dermestes. — This very remarkable hair is ob- 

 tained from the larva of a small beetle, commonly met with in 

 bacon and hams and other dried animal substances; it is 

 covered over with brownish hairs, the longest specimens of 

 which should be selected. When one of these is viewed with 

 a magnifying power of 200 diameters, the upper part presents 

 the appearance shown at C 1, and may be said to consist of a 

 shaft and expanded extremity or head ; the shaft, like that of 

 the hairs of some other larvae, is covered with whorls of large 

 close-set spines four or five in number in each whorl ; these 

 are closely arranged one above the other, as seen at 2 ; the 

 upper part of the shaft, near the head, is provided with several 

 larger and more obtuse spines, forming a knob above this, as 

 seen in 1 and 3 ; the shaft is naked for a very short distance ; 

 it then becomes invested with six or seven large filaments or 

 spines, which are pointed at their distal extremities, and pro- 

 vided with a small protuberance at their proximal ends, where, 

 by slight pressure, they may be separated one from the other, 

 as seen at 3, or they may sometimes be detached at the apex, 

 as seen at 1. In the early days of testing microscopes, these 

 hairs were found rather difiicult of definition, and no one 

 would imagine that fig. 20, in Mr. Pritchard's twelfth plate, 

 before quoted, was of the same nature as C 1, 2, 3, in plate vi. 

 of the present work. This very beautiful hair now forms a 

 good test of the defining power of a half-inch object-glass. 



We next come to a class of objects much more difficult to 

 exhibit than any of the preceding; these wiU form excellent 

 tests of the good qualities of the quarter and one-eighth of 

 an inch object-glasses, and consist of scales removed either 

 from the wings or the body of insects. 



