MAGNIFYING POWERS. 



161 



fering with definition. The spherical aberration may be con- 

 siderably diminished by attending to the figm^e of the lens 

 employed ; thus, if it be a planoconvex, the convex side should 

 be placed towards the eye, if a double convex, it has been 

 found in practice that one whose radii are in the proportion of 

 one to six is the form in which the aberration is the least ; but 

 it can be entirely got rid of by combinations of lenses so dis- 

 posed, that their opposite aberrations may correct each other ; 

 this was first accomplished in a satisfactory manner by the 

 doublet of Dr. Wollaston, before described in pages 30 and 

 65, as consisting of two planoconvex lenses whose focal lengths 

 are in the proportion of one to three, the lens of shortest focus 

 being placed next the object, and the convex surfaces of both 

 directed towards the eye, with a stop or diaphragm between 

 them. "Wollaston did not employ the stop, as his doublets were 

 of such high power, that the lenses nearly touched each other. 

 The action of the doublet wiU be best understood by the 

 diagram, fig. 114, copied from Mr. Eoss's article " Microscope," 



1 before alluded to, where 

 P represents a portion of 

 the pupil, D D the dia- 

 phragm or stop, and L O 

 L' the object. Each of the 

 pencils of light from the 

 extremities of the object 

 L L' is rendered eccentric 

 by the stop, and the ray 

 that passed through the 

 first lens near to the cen- 

 tre is made to pass through 

 the periphery of the se- 

 cond lens, and on the op- 

 posite side of the common 

 axis, P O; thus each is 

 affected by opposite errors 

 which, in some measure, 

 "neutralise each other, and 

 the rays, R B, R B, emanating from L, being bent to the 

 11 



