SIMPLE MICROSCOPES. 



the object can be placed at a much greater distance from the 

 anterior lens D D, and can consequently be more conveniently 



Fig. 13. 



manipulated, if it be desired to dissect it, or to submit it to any 

 other process ; it can also be illuminated by a light thrown upon 

 that side of it which is presented to the glass ; this could not be 

 done if it were nearly in contact with the glass, which must 

 necessarily be the case by reason of its very short focal length, if 

 a single lens were used. 



30. It was recommended by Dr. Wollaston, tho inventor of 

 these doublets, to give the two lenses com- 

 posing them unequal focal lengths, that of 



E E being three times that of D D. 



The lenses are usually set in two thimbles, 

 one of which screws into the other, as shown in 

 fig. 13, so that they can be adjusted as to their 

 mutual distance, so as to produce the best effect. 

 When still higher powers are sought, the lens D D is replaced by 

 two plano-convex lenses, in contact, which taken together play 

 the part of the single lens D D in the doublet ; this combination 

 is called the triplet. 



When a very low magnifying power is required, the lenses E E 

 and D D may be separated, by unscrewing. 



31. The lenses, whether of a doublet or a triplet, being thus 

 properly mounted, expedients must be adopted to enable the 

 observer to apply them conveniently to the object under examina- 



109 



