NOMENCLATURE OF OBJECTIVES. 131 



allowance has been made for this last condition, we 

 have seen English eighths having really higher power 

 than some American one-twelfths. Again, but a few 

 weeks since, we handled a foreign one-sixth which was 

 superior in amplification to a Spencer one-eighth. We, 

 here at home, have scolded a good deal at this state of 

 things, and at times have rated our English cousins for 

 thus underrating their objectives. It is quite unneces- 

 sary here to traverse the ground we have already dis- 

 cussed, to render it evident that as the objective ap- 

 proaches perfection, these nominal distinctions, based 

 on focal length, fail to have particular force. If, for 

 example, it were possible to produce a two-inch objec- 

 tive, which, under extremely high eye-piecing, would 

 more than do the work of our present tenths, then we 

 could afford to work pretty much with one objective, 

 and to the eye-piece look for the determination of the 

 power. Until, however, some such " possibility" shall 

 occur, our present nomenclature will be invested with 

 some force, no matter how variable this force may be. 

 It is, therefore, desirable sometimes to determine the 

 actual rating of an objective. 



The method usually employed, is to place a stage 

 micrometer in position, using a ten-inch tube, and pro- 

 jecting the image by aid of the camera lucida on a screen 

 ten inches distant. Knowing, then, the actual value of 

 the divisions on the micrometer, we are enabled, by the 

 measure of the magnified image thereof, to determine 

 the amplification. This method is so well known and 

 so often practiced, we content ourselves with its gen- 

 eral mention. 



