JOURNAL 



OF THE 



ROYAL MICROSCOPICAL SOCIETY. 



AUGUST 1900. 



TRANSACTIONS OF THE SOCIETY. 



V. — On the " Lag " in Microscopic Vision. 



By Edward M. Nelson. 



{Read 16'A May, 1900.) 



The method employed by the "man in the street" to determine the 

 efficiency of a clock is to test its capability of keeping time ; but a 

 scientific horologist would not be satisfied with this mere time-keeping 

 test ; he would want to know the difference between the amount of 

 energy put into the clock and that given out by the clock — in other 

 words, the amount of energy wasted in the clock itself by friction, etc. 

 Example : — The work given out by a common grandfather's clock 

 is the wagging of a 4 lb. pendulum, 40 in. long (roughly), and tbe 

 energy put into the clock is a 10 lb. weight falling 4 in. per diem. 



10 x 4 



The ratio between these quantities is therefore -. — = 4. Com- 



1 4 x 40 



pare this with a good regulator which wags a 30 lb. pendulum 



40 in. long, by means of a 5 lb. weight falling 3 in. per diem ; 



5x3 



thus — r — = ^q ; the regulator therefore has twenty times the 



efficiency of the grandfather's clock. 



Now, we microscopists have hitherto, like " men in the street," 

 been content to test a Microscope objective by measuring its nume- 

 rical aperture, and indicating the nature of its performance by some 

 qualifying adjective ; thus, a microscopist may be heard to say that a 

 certain 1/4 is a beautifully corrected lens of 0"8 N.A., whereas, with 

 regard to another lens of precisely the same power and aperture, he 

 will not commit himself further than to describe the lens as fair, or 

 tolerably good. The question therefore that must present itself to 

 every one's mind is : — Cannot we microscopists, following the example 

 of scientific horologists, substitute, by means of a formula, definite 

 figures for the indefinite terms applied to objectives, such as excellent, 

 fair, tolerably good, etc. ? The scope of this paper then is to show how 

 this may be easily accomplished. The method is as follows : — First, 

 find out the tangent of the visual angle for a minimum visible with 



Aug. 15th, 1900 . .. . . 2 F 



