THe Microscorr. at 
the work done by others, I have taken the second mm. of “Cen- 
timeter Scale A, 1883,” which according to Prof. Rogers when 
defined by the central defining lines needs no correction, and 
compared it with the third mm. of the same scale, which accord- 
ing to Prof. Rogers is 0,36 mikron longer than the second mm., 
using a 1" Zeiss objective, so as to bring the whole length fairly 
within the field. With the glass eye-piece micrometer the sec- 
ond and third millimeters seem to be of exactly the same length, or 
if there is any difference itis imperceptible, each covers exactly 
197 divisions of this micrometer, and I was particular to use the 
same intervals in making the comparison. With the filar microm- 
eter on the other hand, the difference was at once apparent, the 3d 
millimeter being perceptibly longer than the second at every 
measurement. I will not now say how much longer, for 
the reason that I have not made a series of measurements 
sufficiently numerous to warrant me in expressing a decided 
opinion upon this point, and for the further reason that the value 
of a division in each case was determined in the centre of the 
field and the value in the centre is different with all the objec- 
tives I have examined from that at or near the edge of the field. 
I adduce this solely for the purpose of showing that the filar 
micrometer will, with a low power, render apparent differences in 
length which the glass eye-piece micrometer will not. That this 
is so will not seem strange when we remember that the smallest 
divisions of the ordinary eye-piece micrometer are usually not 
less than yoo inch, while one division of most filar micrometers 
is much less. The pitch of the screw of my own is approxi- 
mately } mm. and the index wheel is divided into 200 parts, so 
that one division represents an absolute movement of the spider 
line of about <$5 mm. or yotoo inch. By the aid of alensI read 
each division to tenths. With the unaided eye it requires great 
accuteness of vision and much skill to estimate a fraction of one 
division of the glass micrometer. In fact, I do not think 
the average eye is equal to the task of doing it with 
accuracy under ordinary circumstances. Under favorable 
circumstances one division may be halved by estimation and may 
perhaps be divided into even smaller fractions, though such esti- 
mation partakes so largely of the character of a mere guess as to 
be of little value in precise measurements. I should say then that 
