lioijat Society, 441 



PROCEEDINGS OF LEARNED SOCIETIES. 



ROYAL SOCIETY. 



June 15, 1854. — The Earl of Rosse, President, in the Chair. 



" On the Structure of certain Microscopic Test-objects, and their 

 Action on the Transmitted Rays of Light." By Charles Brooke, 

 M.A., F.R.S. 



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 in- 

 ternal 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 struc- 

 ture of two well-known test-objects, the scales of Podura plumbea, 

 and the siliceous loricse 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 latter as the tests of the penetrating or sepa- 

 rating power as it has been termed. 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 correc- 

 tion is never theoretically perfect, since there will always be residual 

 terms in the general expression for the aberration, whatever prac- 

 ticable 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 a very delicate transparent lamina or membrane, covered with an 

 imbricated 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 recognized. This structure appears to be ren- 

 dered 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 recognized when 

 the scale is seen as an opake object under a Ross's yVth (specially 

 adjusted for uncovered objects), illuminated by a combination of the 



