162 



by two refracting surfaces, or by .1 single lens of a certain thick- 

 ness. 



The author computes the relations of tlie radii of curvature of 

 such a lens, and the interval between its surfaces, so as to fulfil — 

 1. the condition of achromatism ; and, 2. that of the whole light 

 entering the object-glass being received also by the eye. This se- 

 cond condition determines the radii in the ratio of the line of in- 

 cidence to the line of refraction. The correction for sphericity can- 

 not be also included on this construction. 



