72 On the Similarity between the Red Blood-corpuscles of Man 
Of course, in arranging for these measurements, the effect of 
the screw-collar adjustment of the objective on the magnifying 
power had to be taken into account. This was done in the following 
manner : some thin glass covers, not varying more than a thou- 
sandth of an inch from *012 inch in thickness, were selected from 
a lot of so-called ^fo^th inch covers by means of a suitable lever of 
contact.* Some blood being placed under one of these covers, 
the best adjustment of the screw-collar for definition was found by 
trial. The stage-micrometer, which is an uncovered one, was then 
temporarily covered with another of the selected thin glasses, and 
being duly focussed upon the desired value was given to the 
divisions of the eye-piece micrometer by the adjustment of the 
draw-tube, after which the measurements were proceeded with, 
and the screw-collar was not turned again until they were com- 
pleted. 
The following tables present the several means deduced from 
these measurements in decimals of an inch, to which, for con- 
venience, I have added the equivalent values in decimals of a milli- 
meter. The number of corpuscles from which each mean was 
deduced is also given. The measurements made with the Hartnack 
No. 13 immersion are marked (H), the others were made with 
Powell and Lealand's immersion -Y^th. : 
Measurements of Human Red Blood-corpuscles, from Five Individuals. 
Number of 
Corpuscles 
Measured. 
1. Dr. W., dry .. 
2. Do. moist .. 
3. Do. do. (H) 
4. Do. do. (H) 
5. Dr. McC, dry .. 
6. Do. do. .. 
7. Do. moist .. 
8. Mr. W., dry .. 
9. Do. do. (H) 
10. Mr. T. do. .. 
11. Do. do. (H) 
12. Mr. B. do. .. 
13. Do. do. (H) 
50 
49 
50 
50 
50 
50 
50 
50 
52 
50 
50 
50 
50 
Mean Diameter. 
Decimal of 
an English Inch. 
■000304 
•000292 
•000300 
•000289 
•000288 
•000294 
•000301 
•000298 
•000297 
•000290 
•000292 
•000296 
•000297 
Decimal of 
a Millimeter. 
•00772 
•00742 
•00762 
•00734 
•00731 
•00747 
•00765 
•00757 
•00754 
•00737 
•00742 
•00752 
•00754 
In each of these measurements of human blood the great 
majority of the corpuscles ranged from twelve to seventeen divisions 
of the eye-piece micrometer ; that is, from • 00024 to • 00034 of 
an inch. Out of the whole number measured, six were as small as 
* The instrument used was made by Stackpole and Brothers, of New York, 
after the pattern of the instrument designed by Mr. Eoss, which is figured in 
' Carpenter on the Microscope,' 4th edit., London, 1868, p. 203. 
