120 THE AMERICAN MONTHLY [July 



I think, made one witb wide . aplanatic cone. Today 

 every English ht>use of any standing constructs achro- 

 matic combinations with 0*90 of aplanatic cone, and two 

 construct apochromatics. Not long ago, having the 

 opportunity of testing and comparing three similar 

 objectives together, I was enabled to see the difference. 

 With the Abbe condenser there was no very obvious dis- 

 tinction ; but tested by English condensers it was quite 

 otherwise. The great firm had no cause to blush for any 

 one of them ; all were good lenses ; but they now revealed 

 as distinct characteristic features as one sees in indi- 

 vidual faces. On a graduated series of Poduras, one of 

 them now gave most unusually good definition with 

 rather a small cone under the highest ( X 27) eyepiece; 

 while a second, scarcely equal in this point, excelled the 

 others in the toide cone it was able to use on this object. 

 Another operator more skillful than myself, and certainly 

 of keener vision quite independently reached identical 

 conclusions. Slight variations of pressure in the final 

 polishing of the glasses are quite sufficient to produce 

 such differences as these, in such small lenses as are 

 here in question. 



Whether this latter be the cause, or some other, 

 nearly all high power objectives even of the present day, 

 and of the very best makers, show a very sensible 

 amount of aberration. Drawing a circle to represent 

 the whole aperture, and smaller concentric circles to 

 define zones of its surface, many of the zones have 

 slightly different foci. This fact plays all sorts of insid- 

 ious hanky-panky-tricks with small-cone interference 

 images of the Abbe kind; giving more force to such of 

 the spectra as are correctly focussed than to the others. 

 But in other respects, with small cones, these zonal dif- 

 ferences are not obvious, and often escape detection, 

 many portions of the aperture not being utilized at all. 

 There are refined tests familiar to opticians, and some 



