n6 ENTOMOLOGICAL NEWS. [May, 



ENTOMOLOGICAL MEASUREMENTS. 



By THOMAS L. CASEY. 



The dimensional measures published by the writer up to the 

 present time have been obtained by a simple scale and the unaided 

 eye, fractional parts of the unit being estimated. The most con- 

 venient unit, even for division into tenths, has proved to be the 

 single millimeter. Very lately, however, Prof. W. A. Rogers, of 

 Waterville, Me., has ruled for me several scales in half millime- 

 ters on thin glass for use as a micrometer, being laid for this pur- 

 pose upon the diaphragm within the eye-piece. The value of a 

 division of this micrometer with the i^-inch objective I find to 

 be .12 mm. It occurred to me to measure some types with this 

 micrometer in order to practically compare the results with those 

 given, for example, in my recent monograph of the Scydmaenidae 

 (Col. Not. vii). The only three types which happened to be at 

 hand at the time were those mentioned below; they were sepa- 

 rated from my cabinet for a special purpose and without reference 

 at all to a revision of their estimated dimensions, and may there- 

 fore be regarded as a fair average of all the measures given in 

 the monograph. The differences between the lengths and widths 

 as observed by the unaided eye and scale, and the true dimen- 

 sions as taken from the micrometer are as follows : 



LENGTH DIFF. WIDTH. DIFF. 



Obs. True. mm. Obs. True. mm. 



Noctophus schnritti ... 2.1 2.16 .06 0.78 0.84 .06 



Eutheia americana . . . i.o 1.08 .08 0.38 0.44 .06 



Ver aphis capitata ... 1.25 1.34 .09 0.5 0.56 .06 



It seemed difficult at first to account for the virtually constant 

 personal equation, \vhich is evident at once from these results, 

 until it occurred to me that in measuring with the scale and 

 unaided eye, the scale could never be brought quite into the plane 

 of the insect to be measured, but was always nearer to the eye 

 by a very small but sensible distance, and therefore seen under 

 a larger visual angle than the object; the dimension of the latter, 

 as thus obtained, was therefore, of course, relatively too small 

 by an amount corresponding to the difference in distance from 

 the eye of the scale and insect. This amount is practically .06 

 mm., and it would be safe to increase all measurements hitherto 

 given by the writer by this amount, but in future I shall use the 

 micrometer for insects under 2 mm., or thereabout, in size. The 



