84 SEM-OflON AND USE 



Testing Objectives. -At first sight it would seem to be 

 the easiest thing in the world to test an objective, and find out 

 whether or not it is capable of doing certain work, but a little 

 experience soon teaches those who are not too self-conceited, 

 that it is the easiest thing in the world to be deceived. We 

 have seen those who considered themselves the most capable 

 of judges, condemn lenses that had received the approbation of 

 the ablest microscopists in the world lenses too that had shown 

 their efficiency by doing really good work; showing that even 

 those who consider themselves very expert, may sometimes arrive 

 at wrong conclusions. If this is the case, then, with men of 

 training and experience, how can a beginner, who has had no 

 experience, hope to be able to form a correct judgment in re- 

 gard to the quality of an objective ? 



But while it is difficult, or perhaps impossible, to pronounce 

 a positive opinion in regard to the quality of an objective, 

 especially those made for some of the higher departments of 

 microscopic work, it is in general easy for those who have had 

 experience, to form a judgment in regard to ordinary objectives, 

 or at least those designed for ordinary purposes. The ability 

 to form such a judgment depends rather upon experience and 

 a comparison with the work of other glasses than upon a refer- 

 ence to any special standard; and therefore, as a general rule, 

 we would advise beginners who are about to purchase objectives, 

 to obtain the advice and assistance of some skilful friend. 

 To those who cannot obtain such assistance, we offer the follow- 

 ing hints. 



The great difficulty in the way of arriving at a decision in 

 regard to the quality of an objective, is the want of a standard 

 with which to judge its performance. When we examine the 

 image which an objective gives of any object, it is very difficult 

 to decide whether or not that image truly represents the ob- 

 ject. Take, for example, the podura scale: wide differences of 

 opinion exist as to its structure, and how it ought to look; sup- 

 pose, then, that two objectives show entirely different appear- 

 ances of this object, who shall decide which one is correct ? 

 And if, even in the case of expert microscopists, this holds 

 true absolutely, which it does, how shall a beginner determine 

 that the images which he sees through an objective are true or 



