48 SELECTION AND USE 



angular aperture and penetration must of necessity be in in- 

 verse ratio to each other, the Boston school claim to have pro- 

 duced objectives of very high angles, which at the same time 

 possess great penetration. Even the definition of angular 

 aperture seems to be unsettled, all of which is, of course, very 

 puzzling to a beginner. In the present work we propose to 

 accept the old definition, and to confine ourselves to generally 

 acknowledged facts in regard to the other points. 



Fig. 3. 



It was Dr. Goring, we believe, that first pointed out the 

 special advantages of high angles, and suggested the use of test 



