On the "Lag" in Microscopic Vision. By E. M. Nelson. 585 



In this case, the O.I. was found to be 19, therefore 

 m -t - O.I = 450 - 8 - 19 = 423, 



and the " order of merit " is 



4370 



42; 



= 10-3, 



If any one desires to avoid the empiricism in the last formula,, 

 and prefers the simple theoretical value, then the formula 



tan m 



should be used. This in the above example is 



4370 

 450 



= 9-7. 



In these experiments the illumination for the non-achromatic 

 objectives in the first group was a full axial solid cone, and for the 

 Tulley achromatics in the second group a % cone was employed. 



The plate was ruled by Mr. H. J. Grayson, of Melbourne, and 

 was mounted in realgar of 2 • 5 refractive index. 



