24 



PROPERTIES OP LENSES. 



changed into a virtual image at a' b' by the latter, 

 both images being inverted as regards the object. 

 The principle of the more complex forms of the com- 

 pound microscope is similar, the number and arrange- 

 ment of the lenses being different both for objective 

 and eye-piece, the former has been already explained. 

 The eye-piece generally used is the Huygeian 

 named after its inventor Huygens, it consists of two 

 piano convex lenses, 1 and 2 fig. 14, called respec- 



Fig. 14. 



tively eye and field lens, the former being one-third 

 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 possessed, 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. 



We advise the amateur not to use it in photo- 

 graphy except to view the image before focussing, 

 as with all its supposed advantages its employment 

 never gives good results. 



