4o6 
HOWARD E. PULLING 
of focus. An object will, accordingly, be in focus only when there 
is no movement of the image as the diaphragm is moved back and 
forth. It is unnecessary, except for completeness, to direct attention 
to the importance of maintaining a "critical illumination."^ 
With a Leitz No. 2 ocular and a 1/12 oil-immersion objective, the 
angular micrometer as constructed has a sensitiveness of 0.000009 
mm., i. e., 3. movement of the protractoi through i' of arc would move 
the pointer-tip over the image of an arc whose chord is 0.000009 mm. 
The use of higher powers increases the sensitiveness as does also placing 
the pointer-hair near the periphery of the field. Both are obviously 
unnecessary so far as sensitiveness is concerned. 
Farmer and Digby^ have observed that it was impossible for them 
to measure chromosomes with an accuracy greater than 0.0001 mm., 
and this number may be taken as the limit of accuracy of former 
methods of measurement. Measurements were made with an angular 
micrometer upon an individual somatic chromosome of Chinese Lily, 
during anaphase, with about half the magnification of the English 
authors, and consequent loss of accuracy. They were made at dif- 
ferent times on different days, so that it is believed they offer a fair 
test of the efficiency of the instrument. In nineteen measurements 
the maximum deviation from their mean was 0.00003 nim. and the 
difference between the extremes was 0.00006 mm. The maximum 
variation is therefore about six-tenths that found by Farmer and 
Digby, and would of course be still further decreased with increased 
magnification since this would increase the accuracy with which 
pointings might be made. 
It is a pleasure to acknowledge my indebtedness to Dr. C. E. Allen 
for his help and interest. 
University of Wisconsin, 
Madison, Wisconsin 
^ See the discussion in some good work on the microscope such as Spitta, E. J., 
Microscopy. London, 1907. 468 pp. 
^ Farmer, J. B., and L. Digby, On dimensions of chromosomes considered in 
relation to phylogeny. Phil. Trans. Roy. Soc. London, B, 205: 1-25. 1914. 
