of two double convex lenses, separated by a black 

 diaphragm, giving the external appearance of a Cod- 

 dington while in reality being only an inferior Doublet. 



Aplanatic Triplet. The best type of magnifier 

 is the Aplanatic Triplet, Fig. 18. It is composed of 

 three lenses cemented together in such a manner that 

 they are virtually one. For many years this form was, 



Fit. 18. Fig. 19. 



on account of cost, used only to a limited extent, but 

 an increased demand has brought about a greatly 

 reduced price. Its advantage lies in the fact that both 

 the spherical and chromatic aberrations are corrected 

 to a high degree, thus enabling the observation of 

 minute detail and giving a large field which is flat to 

 the extreme margin. It can be highly recommended 

 to those wishing a good magnifier. 



Hastings' Aplanatic Triplet. This form of 

 lens, Fig. 19, has been computed by Prof. C. S. Hastings 

 of Yale University and is a modification of the Aplanatic 

 Triplet, giving not only the highest spherical and chro- 



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