107 



comes 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 considered to be the 

 case. While in objectives of the same aperture it 

 may vary considerably, it may in others of differ- 

 ent aperture be so that the higher one may have 

 the greater working 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 dis- 

 tance 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 single 

 lens of the same magnifying power. 



As may be imagined, there is a variety of 

 opinions as to what constitutes long or short work- 

 ing distance 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. 

 It has several times occurred in the experience of 

 the writer, that objectives were complained of as 

 having no working distance (that the objective 

 could not be focused) when on investigation it was 

 found that window glass or a slide had been used 

 for a cover. 



To Measure Working Distance. The actual 

 or available working distance is of little moment 

 except in objectives of medium or high power and 



