30 



THE MICROSCOPIST. 



best arrangement of this kind is Dr. Wbllaston's Doublet, 

 which consists of two plano-convex lenses, whose focal lengths 

 are as 1 to 3 ; the plane sides of each, and the smallest lens, 

 placed towards the object. The lenses are set in separate cells 

 so as to adjust their proper distance apart, which is best done 

 by experimenting on their performance, although their distance 

 is about the difference of their focal lengths. Between them is 

 a diaphragm or stop, generally placed immediately behind the 



Fig. 7. 



anterior lens. The stop was not employed by Dr. Wollaston, 

 as his lenses were of such high power that they nearly touched 



