THE DOUBLET. 37 



excentrical by the diaphragm d, d ; consequently, they 

 pass through the lenses on opposite sides of their 

 common axis o p; thus each becomes affected by 

 opposite errors, which to some extent balance and 

 correct each other. To take the pencil Z, for instance, 

 which enters the eye at r b, r I : it is bent to the right 



at the first lens, and to the left at the second ; and 

 as each bending alters the direction of the blue ray 

 more than the red, and, moreover, as the blue ray 

 falls nearer the margin of the second lens, where the 

 refraction being more powerful than nearer the centre, 

 compensates in some degree for the greater focal length 

 of the second lens, the blue rays will emerge very 



