24 PROPERTIES OF LENSES. 



the focus of the latter and the distance of their 

 separation half the sum of their focal lengths. 



Huygens was ignorant of one advantage this com- 

 bination possesses, he adopted it simply to remedy 

 spherical aberration, but it has been since shown 

 that it corrects chromatic aberration also. A refer- 

 ence to the figure will explain the principle on which 

 the formation of the image depends. 



It should never be used in photography except 

 to view the image before focussing, as with all 

 its supposed advantages its employment never gives 

 good results. 



