TESTING THE OBJECTIVE. 67 



same both within and without the focus. A different 

 indication, however, is afforded by such test-objects as 

 present (like the Podura-scale, the Diatoms, &c.) a 

 set of distinct dots or other markings. If the dots 

 have a tendency to run into lines when the object is 

 without the focus, the glasses should be brought closer 

 together; on the contrary, if the lines appear when 

 the object is within the focal point, the glasses must 

 be farther separated. 



The Podura-scale is an excellent test-object. The 

 structure consists of a delicate transparent lamina or 

 membrane, covered with an imbricated arrangement of 

 epithelial plates, the length of which is six or eight 

 times their breadth, and arranged like the tiles on 

 a roof, or the long pile of some kinds of plush. The 

 scales may be readily obtained by putting a live 

 Podura into a small test-tube, and inverting it on a 

 glass-slide; the insect should then be allowed for a 

 time to leap and run about in the confined space. By 

 this means the scales will be freely deposited on the 

 glass ; and being subsequently trodden on by the insect, 

 several will be found from which the epithelial plates 

 have been partially rubbed off, and at the margin of 

 the undisturbed portion the form and position of the 

 plates may be readily recognized. Their structure ap- 

 pears to be rendered more palpable by mounting the 

 scales thus obtained in Canada balsam, and illuminating 

 them by means of Wenham's parabolic reflector. The 

 structure may also be very clearly recognized when the 

 scale is seen as an opaque object under a Boss's -^ (spe- 

 cially adjusted for uncovered objects), illuminated by 

 a combination of the parabola and a flat Lieberkuhn. 

 The under-side of the scale appears as a smooth glis- 

 tening surface, with very slight markings, correspond- 

 ing, probably, to the points of insertion of the plates on 

 the contrary side. The minuteness and close proximity 

 of the epithelial plates may account for their being 

 found a erood test of definition, while their prominence 

 renders them independent of the separating power due 

 to larger aperture. 



The structure of another class of test-objects, the 

 F 2 



