AMERICAN 
JOURNAL OF BOTANY 
Vol. Ill October, 1916 No. 8 
THE ANGULAR MICROMETER AND ITS USE IN DELICATE 
AND ACCURATE MICROSCOPIC MEASUREMENTS 
Howard E. Pulling 
In the formulation of a cytological problem that necessitated 
the accurate measurement of extremely small bodies it became evident 
that the commercial ocular micrometers were neither sensitive nor 
accurate to the required degree. Although it is generally recognized 
that any instrument whose precision depends upon the accuracy and 
lack of variation in the pitch of a screw is subject to large errors of 
construction and to further errors resulting from wear upon the screw 
and its bearings, it is often believed that the best filar micrometers 
are sufficiently accurate for all general needs of the microscopic. 
That this conclusion is not warranted unless each instrument is sys- 
tematically investigated for error and the necessary corrections made 
has been shown by Gray,^ who points out further that satisfactory 
instruments may easily be rendered worthless by dust particles that 
grind the screws unevenly and that in all cases instruments must be 
frequently investigated for error. 
That such general observations as these are of interest not only 
to workers who demand the very highest degree of accuracy from 
micrometers but that they affect all microscopists who use such 
instruments is evidenced by the following example drawn from my own 
experience. A filar micrometer, one of the best types obtainable, 
was used to measure the same fixed distance over different portions 
of the screw. Through the exercise of great care in the selection of 
the object and in the manipulation of the instrument the readings 
made on any one position of the object did not vary from the mean 
of the readings for that position by more than 1/500 of a drum revolu- 
1 Gray, Arthur W., Micrometer Microscopes. Scientific paper of the Bureau 
of Standards No. 215 (reprint of Bulletin of the Bureau of Standards, 10^ 375-390. 
November 5, 1913). Washington, D. C, 1914. 
393 
