58 CONSTRUCTION OF THE MICROSCOPE. 



freed from organic matter : the former of these is com- 

 monly adopted as the test of the defining power of an 

 achromatic object-glass, and the several species of the 

 latter as the tests of the 'penetrating or separating power, 

 as it has been termed. The defining power depends only 

 on the due correction of chromatic and spherical aberra- 

 tions, so that the image of any point of an object formed 

 on the retina may not overlap and confuse the images of 

 adjacent points. This correction is never theoretically 

 perfect, since there will always be residual terms in the 

 general expression for the aberration, whatever practicable 

 number of surfaces we may introduce as arbitrary con- 

 stants; but it is practically perfect when the residual 

 error is a quantity less than that which the eye can appre- 

 ciate. The separation of the markings of the Pleurosig- 

 mata and other analogous objects is found to depend on 

 good defining power associated with large angle of 

 aperture. 



The Podura scale appears to be a compound structure, 

 consisting of a very delicate transparent lamina or mem- 

 brane, covered with an imbricated arrangement of epi- 

 thelial plates, the length of which is six or eight times 

 their breadth, somewhat resembling the tiles on a roof, or 

 the long pile of some kinds of plush. This structure may 

 be readily shown by putting a live Podura into a small 

 test-tube, and inverting it on a glass-slide ; the insect 

 should then be allowed for some time to leap and run 

 about in the confined space. By this means the scales 

 will be freely deposited on the glass ; and being subse- 

 quently 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 recognised. 

 This structure appears to be rendered most evident by 

 mounting the scales thus obtained in Canada balsam, and 

 illuminating them by means of Wenham's parabolic re- 

 flector. The structure may also be very clearly recognised 

 when the scale is seen as an opaque object under a Ross's 

 ■j^th (specially adjusted for uncovered objects), illuminated 

 by a combination of the parabola and a flat Lieberkuhn. 

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



