398 HOW TO SEE WITH THE MICROSCOPE. 



Now I hold as follows : We will take, for example, two glasses, 

 both possessing the same amplifications and the same apertures ; 

 now of these two glasses, the one having the greater working 

 distance will exhibit structures situated slightly out of the 

 precise focal plane, or what amounts to the same thing, will be 

 less susceptible to slight changes in the focal distance, or again, 

 to use the words of Dr. Carpenter, will have the greater pene- 

 tration. In extreme cases, the item of aperture may be dis- 

 regarded. For instance, as stated in my Dunkirk lecture, a 

 spectacle lens of sixty inches focal length will be endowed with 

 greater penetration than a ToUes duplex objective. 



But in the Dunkirk address, I did not avail myself fully of 

 the demonstration afforded by the spectacle lens, but now 

 invite Mr. Lapham's attention, thus : If it so be that with the 

 spectacle lens I am enabled to see with perfect clearness of 

 vision objects across the road, it will surely occur that when I 

 .attempt to read fine print, that the lens will defeat me. I will, 

 to be sure, see the lines of the print, will recognize the contour 

 of the book, and larger objects in immediate proximity, etc. 

 Here is an example from everyday life, to-wit : The glasses 

 suitable for reading will not answer for observations at a dis- 

 tance. True it is that with such glasses, with which we read 

 with ease, we may also see the general forms of things across 

 the road, yet they suffice not to render clearly the details thereof. 

 Hence we have all of us observed that many persons wearing 

 convex glasses often use two pairs of spectacles, one for near 

 and one for distant vision.- 



Now in this everyday case we are taught a lesson, that, from 

 some cause or other, has been a slow one to acquire. First 

 *' penetration," in its naked aspect, is simply a function depend- 

 ing upon working distance; and secondly that "penetration," 

 wtless accompanied with a certain amount of definition* is practi- 

 cally worthless. I ask Mr. Lapham's close attention to this point, 

 one that has been terribly overlooked. 



Now, if Mr. Lapham's one-sixth of 180 happens to just 

 ** turn the corner," i. e. have balsam angle, say, about 85, I 

 would feel sure that a little observation would lead him to the 

 same opinions I have arrived at, and I only fear that he has 



