4 JOURNAL OF THE [January, 



AMPLIFICATION IN MICROMETRY. 



BY HON. MARSHALL D. EWELL, LL. D. 

 {Read December 20th, 1889.) 



My attention has quite recently been drawn to this subject in 

 connection with the celebrated " Dr. Cronin case." It may be 

 taken for granted that one cannot measure what he cannot see- 

 But how high an amplification is necessary in a given case is a 

 matter of much importance. In the measurement of blood- 

 corpuscles in medico-legal cases the late Dr. Richardson advo- 

 cated the use of a very high power, viz.: a h or stj objective. 

 In my own measurements of blood-corpuscles I have out of re- 

 spect to authority, always used a high power, from 1,500 to 1,800 

 diameters. Recent experience has, however, qualified my views 

 upon the subject, and in the case of the comparison of the ulti- 

 mate subdivisions of a micrometer, ruled on metal, I am now of 

 the opinion, that practically the same result may be obtained by 

 the use of a i objective as with a tV or sV. 



In December, 1885, I commenced the investigation of the 

 xho nim. spaces of " Centimeter A." ; but was unable to finish it. 

 Two series of measurements were then made with a Bausch & 

 Lomb opaque illuminating objective, and a Bullock filar 

 micrometer. Recently I have measured the same spaces with a 

 Spencer to and sV, and with a Zeiss rV. The results of these 

 measurements are given in the table below, each correction being 

 the mean of from three to twelve readings of the filar micrometer 

 at each end of the measured space. 



It will be observed that the agreement between the several 

 series of the writer and the results obtained by Prof. Hilgard is 

 quite close, the discrepancy being practically insensible. 



Provided the amplification is sufficient to render the object to 

 be measured of a sensible size, and to render the difference 

 between the sizes of two objects visible, my own judgment is 

 that little, if anything, is gained by the use of a power so high as 

 to impair the definition, even though such impairment be but 

 slight. Quite as much, in other words, is lost by impairment of 

 definition as is gained by increase of amplification. The prac- 

 tical conclusion then is that no higher power should be used 

 than is consistent with perfect definition. 



