74 ACHROMATISM OF EYE-PIECES, &c. 



Now Mr Airy has shown that, (adopting the notation of this 

 paper) 



a; being the distance of the object-glass from the diaphragm, and 

 p Q its vergency, or the reciprocal of its focal length. Putting 

 for R its value, we get at once xp Q = 



divided by 



[1 + a [ft + p t + p^ + a t [> 2 + p J 



which is identical with his result. 



