SOMETHING FURTHER ABOUT OBJECTIVES. 121 



tration) in the making up of the wider cone as can 

 occur with the use of the narrow aperture ; furthermore, 

 that it would be not only possible, but eminently prac- 

 ticable, by the use of a diaphragm, to cut down the 

 cone of the wider aperture objective to correspond with 

 that of the low-angled glass ; hence it is obvious that in 

 this latter case the two objectives would be worked 

 under similar conditions as respects the angle at the 

 crossing of the rays, and, applying the argument 

 based thereon, neither glass can be endowed with the 

 greater penetration. 



Says one, "How about the working- distance?" 

 The relations of angular aperture to working dis- 

 tance have already been discussed, and intentionally, 

 with the view of preparing the mind of the reader for 

 the above interrogatory. But there remain other con- 

 siderations bearing on the matter of working distance, 

 and the clinching- argument on the part of the writer 

 remains to be presented. 



In doing this, the author is compelled to deal in 

 assertions dogmatically. In the handling and compar- 

 ing of object-glasses, he has had a very large experience, 

 and he feels that he has the same liberty to speak! ex 

 cathedra as has been granted to others. Moreover, 

 what he now has to say is " important if true," and he 

 is as well assured ot their correctness as of any other 

 fact within his knowledge and experience, nor is he 

 alone in the matter about to be stated. Without 

 exception, all who have experimented in the proper 

 direction assent to all that will be here claimed, while 



