88 SELECTION AND THl] 



To determine the quality of an objective it is best to take up 

 in succession the several features which we have just detailed, 

 and examine its efficiency in each of these directions. First of 

 all, the denning power should be carefully tried, this being the 

 most important quality that a glass can possess. No special 

 test can be named for this, and in fact the formation of a cor- 

 rect judgment in regard to it will depend more upon the 

 experience of the observer than upon any particular rules that 

 can be laid down. As Carpenter well says: " An experienced 

 microscopist will judge of the denning power of a lens by the 

 quality of the image which it gives of almost any object with 

 which he may be familiar." To which we may add that the 

 inexperienced microscopist will in general fail to detect a want 

 of defining power, no matter what object he may examine. 



The chief points seem to be that the outlines should be sharp 

 and clear, the blacks black, and the other natural colors clear 

 and distinct. Frey compares the image given by a good lens 

 to a good copper plate, or a print with sharp letters, and no 

 illustration could be more to the point. He also states that an 

 objective which is deficient in this respect, is best tested with 

 a pretty strong eye-piece. In our own experience we have 

 found no surer test of excellence than this ; an objective which 

 is deficient in defining power, is sure to "break down" under 

 a high eye-piece. Deep eye-piecing does not effect the resolv- 

 ing power of a lens to the samo extent that the defining power 

 is lessened, and, therefore, the fact that a glass shows lines 

 under a high eye-piece, is not an absolute demonstration of its 

 excellence as regards definition. At the same time, it will be 

 found that considerable angular aperture is absolutely neces- 

 sary to enable any glass to bear deep eye-piecing, because with- 

 out this, the loss of light is so great that nothing can be seen 

 clearly. Hence the truth of the somewhat paradoxical state- 

 ment, that an objective may be really good under a low eye- 

 piece, and yet fail under a high one. 



With the English opticians a favorite test for definition is 

 the Podura scale. Unfortunately, however, the structure of 

 this scale and even the identification of the scale itself, seem 

 to be a matter of doubt. Page after page has been written for 

 the purpose of showing how the Podura scale ought to look, and 



