XX. 0)1 the Improvement of the Microscope. 

 B\ H. CODDINGTON, MA. F.R.S. 



FELLOW OP TRINITY COLLEGE, AND OF THE CAMBRIDGE PHILOSOPHICAL SOCIETY. 



[Read March 22, 1830.] 



Among the numerous excellent suggestions which Dr. Brew- 

 ster has from time to time thrown out to those engaged in the 

 theory or the practice of Optics, there is one which appears to 

 have been most unworthily, and most unaccountably, neglected. 

 It is that of substituting a sphere for a lens in the construction 

 of a microscope. This is the more surprising, as many persons 

 of great eminence have, of late years, turned their attention to 

 the improvement of this instrument, in which pursuit they have 

 spared neither time, labor, nor expense. The only reason which 

 I can give for this is, that as, until the investigations of Professor 

 Airy, which are contained in the present volume of the Trans- 

 actions of this Society, nobody, with the exceptions of Dr. Young 

 and Dr. Wollaston, ever dared to approach the thorny subject 

 of the oblique refraction* of a pencil of rays by a lens, almost 



* I allude liere to llie refraction of light, considered as homogeneous. The cliro- 

 matic dispersion introduces another source of error, which has been very successfully over- 

 come. I have found the colour most completely corrected in microscopes which were, 

 in other respects, so ill constructed, as to be nearly useless. 



