40 



distance. The skill of the optician must in a consid- 

 erable manner determine the amount of it. 



It will be seen from the above that working dis. 

 tance stands in no direct relation to the focal distance 

 of the objective, neither to its nomenclature or rat- 

 ing, and, it may be added, that it is never as great as 

 the focal distance of a single lens of the same magni- 

 fying power. 



As may be imagined, there are a variety of opin- 

 ions as to what constitutes long or short working dis- 

 tance in a certain objective. No definite rule can be 

 laid down for this, as it is conditioned, by the skill 

 and requirements of the manipulator. Although it is 

 an important factor, the idea that it should in all 

 cases be as great as possible, is erroneous, for, while 

 it may be true in a dry objective, it may be the cause 

 of annoyance in one with immersion. On several 

 occasions it occurred in the experience of the writer 

 that after an objective had been completed, it was 

 found that its working distance was so large that the 

 immersion fluid would run out from between the 

 objective and the cover-glass when the instrument 

 was inclined, and it was necessary to change the 

 objective with a view to decreasing its working dis- 

 tance, in order to allow its convenient use. 



Magnifying Power. This is a question, of vital 

 importance in a microscope, not so much as a quality 

 for itself, as in connection with the resolving power. 

 The inquiry should not be simply how many diame- 

 ters an instrument will magnify, but what the precision 



