TEST OBJECTS. 471 



but many persons now use specimens of infusoria of the 

 genus Navicula for the same purpose. Two of the scales 

 from the body are represented by figs. 4 and 5, as seen under 

 a magnifying power of 500 diameters ; that shown at fig. 4 is 

 one of the largest that could be procured ; whilst that at fig. 

 5 is very small, and its markings exceedingly delicate. The 

 surface of each appears covered with immense numbers of 

 delicate wedge-shaped dots or scales, arranged so as to form 

 both longitudinal and transverse wavy markings ; but when a 

 portion of fig. 4 is magnified 1,250 diameters, it presents the 

 appearance shown at fig. 4a ; the scales may then be seen to 

 stand out boldly from the surface ; at the upper part of the 

 specimen they also project beyond the edge. It would appear 

 from Mr. Pritchard's figure, that at the time of the publication 

 of the work above quoted, nothing but longitudinal and ob- 

 lique lines could be made out, the powers then employed not 

 being able to separate the longitudinal ones into a number of 

 very minute elongated dots or scales, and the transverse ones 

 into rows of the same, arranged somewhat in a wavy manner. 

 The smaller scale, fig. 5, is very much more difficult to exhibit 

 than the larger one, and forms a good test of the defining 

 power of a one-twelfth or one-sixteenth ; the markings are of 

 precisely the same nature as those of the larger scale, but are 

 much more difficult to bring out. 



These insects abound in damp cellars, where they may be 

 seen running or skipping upon the walls. Mr. Pritchard 

 recommends the following method of collecting them, viz. : — 

 "To sprinkle a little oatmeal or flour on a piece of black 

 paper, and lay it near their haunts; after a short time 

 the paper may be removed and carefully placed in a glazed 

 bason, so that when they leap from the paper, on being 

 brought into the light, they may fall into the bason, and thus 

 separate themselves from the bait. They should be cautiously 

 handled, and placed either in little tubes or boxes, with cam- 

 phor to preserve them from the ravages of other insects.* 



* It is said that the French opticians employ as a test the scales of a 

 species of Podura, named Petrobius maritimua, which is very abundant on 

 the sea-coast; the markings on these scales are very strong and easily 



