298 E. M. NELSON ON THE MEASUREMENT OF THE 



buys a T *oth objective of 0"65 N.A. Here an optical index of 26 - 

 is implied ; when he gets home he measures it and finds it |rd of 

 055 N.A. with an optical index of only 18*3, or 30 per cent. less. 

 This is not an exceptional case, but one which unfortunately 

 exemplifies the usual practice. Messrs. Zeiss have for long set an 

 excellent example by never sending out lenses below either their 

 catalogued N.A. or shorter foci. I have measured scores of them 

 and have found their optical indices often in excess, and seldom 

 if ever in defect. 



[The method of determining the focal length of an objective, 

 by the indirect method from the magnifying power, may not be 



A 



t 



B 



L too inc\e& _^ 



i 



c 





Mr 



r 



s 



Fig 9. Diagram to show Relative Positions of the Apparatus. 



M Microscope tube. P> Objective. 



A Screw micrometer. C Objective to be measured, in substage. 

 S Microscope stage and micrometer. 



quite clear, hence the following particulars from notes received 

 from Mr. Nelson may be useful. His own words are practically 

 as follows: The microscope is placed horizontally; a low- power 

 objective, 3, 2, or 1| inch, according to circumstances, is placed 

 in position ; screw-micrometer eye-piece ; the objective to be mea- 

 sured is placed in substage, with its front lens facing the stage. 

 A card cut to the pattern as shown in figure (fig. 9) is fixed by 

 means of a clip in front of the window : the card should be 

 placed at the exact measured distance of one hundred inches 

 from the stage of the microscope. 



