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extant, that there are great differences in objectives 

 purporting to be similar, is, in my opinion, not justi- 

 fied, at any rate in the productions of those men who, 

 by general acknowledgement, are at the head of their 

 profession. I admit that, as in everything which 

 depends upon human skill, there is, strictly speaking, 

 no absolute uniformity, but also claim, that with few 

 exceptions, the differences are so slight, that anything 

 but the most expert manipulation cannot detect them. 



It therefore appears to the writer that any informa- 

 tion which will tend to improve the knowledge of 

 testing objectives will not only prove beneficial to the 

 microscopist, but will prove advantageous to the 

 optician, in that his work will receive a fair trial, based 

 upon a knowledge of the principles involved, and 

 that he may be convinced that all his work which 

 deserves commendation will be the better appreciated. 

 The following points are by no means new, but are 

 often lost sight of in making tests. The writer will 

 speak of medium and high power objectives only, as 

 the deleterious influences are most noticable in these, 

 but they apply as well to the lower powers though in 

 a less degree. 



The part of the instrument which has a strong 

 bearing in the performance of the objective is the 

 mirror. It should be adjustable on the mirror-bar, so 

 that it can be accommodated to the variations in dis- 

 tance of the source of light from the instrument. 

 When parallel rays are used, as with light from the 

 sun or clouds, its distance from the object should be 



