In objectives of low power, it is of little considera- 

 tion, but with those of medium and high power, where 

 it must be expressed in o.oi or o.ooi inch, it becomes 

 a matter, of importance. 



Working distance is spoken of as being long or 

 short and varies with the power and angular aperture. 

 Generally the working distance decreases with the 

 increase in numerical aperture and becomes greater as 

 the aperture becomes smaller. It was for a long time 

 considered that these two qualities varied according to 

 a fixed rule, but this at the present time is not con- 

 sidered to be the case. While in objectives of the 

 same aperture it may vary considerably, in others of 

 different aperture the higher one may have the greater 

 w r orking distance. The skill of the optician must, to 

 a considerable degree, determine the amount of it. 



It will be seen from the above that working distance 

 stands in no direct relation to the focal distance of the 

 objective and it may be added that it is never as 

 great as the focal distance of a simple lens of the same 

 magnifying power. 



As may be imagined, there is a variety of opinions 

 as to what constitutes long or short working distance in 

 a certain objective. No definite rule can be laid down 

 for this, as it is conditioned on the skill and require- 

 ments of the manipulator. It has several times occurred 

 in the experience of the writer that objectives were 



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