184 The Microscope. 



micrometer head being divided into 200 parts, which were read 

 to 1-10 of a division. 



The objective used was a homogeneous immersion 1-10, 

 made by H. R. Spencer of Geneva, N. Y., having a numerical 

 aperture of 1.35, and it was used with a Bausch & Lomb achro- 

 matic amplifier, giving an amplification of about 1,500 diame- 

 ters. The immersion fluid used was Prof. Smith's new homo- 

 geneous immersion fluid, the composition of which he has not 

 yet made public. 



The blood was drawn from my finger, and a thin film spread 

 with a needle upon the side of a cover glass from .150 to .165 of 

 a millimeter in thickness, and examined at once, afresh sample 

 being used upon each occasion. It was examined with central 

 illumination, and always under as nearly the same conditions 

 as possible. During the first four days of the examination, I 

 took, night and morning, about one drachm of the elixir of 

 calisaya, iron and strychnia ; during the rest of the time no drug 

 was taken, and the conditions were nearly identical each even- 

 ing. From 25 to 100 corpuscles were examined each evening 

 and I have tabulated the results, giving the smallest, largest, 

 and average size in millionths of an inch, of each 25 corpuscles; 

 also the average of each 50, 75, 100, and 200, corpuscles. The 

 corpuscles were measured large and small, as they presented 

 themselves in the field of the microscope, the only condition 

 being that they should be approximately circular. 



An examination of the following figures shows that the dif- 

 ference between the greatest and smallest averages of 25 corpus- 

 cles is .000,028 or 1-35,714 inch, a magnitude that may be easily 

 measured by any person having the requisite skill and appara- 

 tus. 



The difference between the highest and lowest averages of 50 

 corpuscles, is .000,015 or 1-66,666 inch, which approaches more 

 nearly the limit of micrometric measurement, though probably 

 not beyond it. 



The difference between the highest and lowest averages of 75 

 corpuscles is .000,012 or 1-83,333 inch, which approximates the 

 limit of micrometric measurement. 



The difference between the highest and lowest averages of 

 100 corpuscles is .000,009 or 1-111,111 inch, which is within the 



