30 



Aplanatic Triplet. --The best type of magnifier 

 is the Aplanatic Triplet, Fig. 15, being composed 

 of three lenses cemented together in such a man- 

 ner that they are virtually one. For many years 



Fig. 15, 



this form was, on account of cost, used only to a 

 limited extent, but an increased demand has 

 brought about a greatly reduced price. While it 

 corrects both aberrations it gives a large and flat 

 field and can be commended to those wishing a 

 good magnifier. 



Hastings Triplet.- -This form of lens has been 

 recently computed by Prof. C. S. Hastings of Yale 

 University and is a modification of the Aplanatic 

 Triplet, giving not only the highest spherical and 

 chromatic corrections, but a considerably flatter 

 and larger angular field and longer working dis- 

 tance; ^that is, the distance between lens and 

 object is greater and is an important feature in 

 higher powers, as it admits of better illumination. 



In all of the better types of magnifiers the 



