44 CONSTRUCTION OF THE MICROSCOPE. 



microscope is judged of by the distinctness and comfort with which it 

 exhibits certain objects, the details of which can only be made visible 

 by combinations of lenses of high magnifying power, and a near ap- 

 proach to correctness. Such are called by the microscopist test- objects. 

 Mr. C. Brooke, F.K.S., whose labours have been devoted to the correc- 

 tion of errors which have crept into this part of philosophical research, 

 says : " In order to arrive at any satisfactory conclusions regarding the 

 action of any transparent medium on light, it is necessary to form some 

 definite conceptions regarding the external form and internal structure 

 of the medium. This observation appears to apply in full force to 

 microscopic test-objects ; and for the purposes of the present inquiry, 

 it will suffice to limit our observations to the structure of two well- 

 known test-objects, the scales of Podura plumbea, ar\d the siliceous 

 lories, or valves of the genus Pleurosigma, freed from organic matter : 

 the former of these is commonly adopted as the test of the defining 

 power of an achromatic object-glass, and the several species of the lat- 

 ter as the tests of the penetrating or separating power, as it has been 

 iermed. The defining power depends only on the due correction of 

 chromatic and spherical aberrations, 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 constants ; but it is practically perfect when the residual 

 error is a quantity less than that which the eye can appreciate. The 

 separation of the markings of the Pleurosigmata 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 

 \ very delicate transparent lamina or membrane, covered with an im- 

 yricated arrangement of epithelial 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 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 recognised. 

 This structure appears to be rendered most evident by mounting the 



