June 15, 1854. (Continued.) 

 The EARL of ROSSE, President, in the Chair. 

 The following papers were read : 



XV. " 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., Surgeon of the West- 

 minster Hospital. Received June 1, 1854. 



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 



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