TEST OBJECTS. 115 



consisting of several large filaments or spines, which are 

 pointed at their distal extremities, and provided with a pro- 

 tuberance at their proximal ends. 



This object, with the others above noticed, is a good test of 

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



SCALES OP INSECTS. The dust on the wings and bodies of 

 butterflies, moths, and other insects, prove, on microscopic ex- 

 amination, to be scales or feathers, overlapping each other like 

 the shingles on the roof of a house. They vary much in form 

 and size ; and from the difficulty of developing their structure, 

 they form excellent test objects. In the present list the most 

 easy are first named. 



Lepisma Saccharina. These silvery-scaled insects frequent 

 closets, book-shelves, &c., and are very common. Their scales 

 are very pretty objects, but are so easily made out as hardly to 

 deserve the name of test objects. The longitudinal striae 

 appear to stand out in bold relief, like the ribs of a shell. A 

 good glass should show well the contrast between the striae and 

 the interspaces. 



Morplio Menelaus, The pale blue scales from the upper 

 surface of the wing of this splendid butterfly form a good test 

 for the half-inch object-glass, which should show clearly the 

 transverse as well as the longitudinal striae, giving it a brickwork 

 appearance. If the scale be flat, which is not common, the 

 striae should be seen over the whole surface. Sometimes the 

 scales are damaged, the pigment having been removed ; in such 

 cases the cross striae cannot be seen. The pigment, under 

 very high powers, exhibits a dotted appearance between the 

 striae. 



Tinea Vestianella, or Clothes Moth. The scales of these in- 

 sects are very delicate, and require some tact in the manage- 

 ment of the illumination to resolve their lines distinctly. The 

 small scales from the under side of the wing should be taken ; 

 the others are easy. 



