OBJECT GLASSES. 107 



a wide aperture in hand, see that its work by central or 

 centrally disposed light is not excelled by any objective 

 of narrow angle extant. In calling on the glass at the 

 limits of its aperture, demand, and see to it particularly,, 

 that there be no letting down of general performance,, 

 that the images remain strong and vigorous, that the 

 corrections are not impaired, and even with the widest 

 apertured objective known, that there be no sensible 

 distortion of the image. 



Again, if it so be that you desire thus to test an ob- 

 jective claiming considerable aperture, say 170 or 175% 

 and adjustable, note whether there be any special ad- 

 justment required when worked at or near the limits of 

 its aperture, other than necessary for its correct per- 

 formance by central light; if this be found the case f 

 the indications are that the two sets of pencils are not 

 in harmony with each other as the Germans say, are 

 not " married." It will be necessary to use a little 

 discretion here, for some of the very finest glasses re- 

 quire a slight change of adjustment under the condi- 

 tions named. The less of this especial adjustment,, 

 however, the better. 



Any objective that will stand acceptably the fore- 

 going tests may be allowed to ' < pass muster " as to its 

 aperture sans peur et sans reproche. 



In the act of writing the above, the author was inter- 

 rupted by a friend with the remark, " Are you not 

 screwing things down pretty fine? Don't you see that 

 your method not only is a severe test as to working- 



