Tue Microscope. 39 
length of 8.9 inches from lower end of nose-piece to upper end 
of tube, and a glass eye-piece micrometer with the lines ;,45 inch 
apart, the value of one division of my eye-piece micrometer is 
as follows: 
fame aries, 1-division, ....... _L22225--5----+-h- 5 be 
3-5 ‘ Lett eee eS hE es 3.36 yp. 
1-4 ‘ S. Yony, correction 10?)e3-5.- =--++-< 298. ft. 
1-4 ‘“* Bausch & Lomb, (opaque illuminator)- 1.27 y, 
1-6 ‘* Zeiss, (cov. cor. 10°,) ‘ a =~» pA 4 se, 
1-10‘ Spencer, (cov. cor. 1 rev. + 103°) with 
Bausch & Lomb pmplifier, MY gs2s2-—2--—5 205 
eee fees, Waly... ee seen ne 219 pz. 
From this table it appears that with the one inch Zeiss, a 
difference of from 2.5 ». to 5 uw. is the smallest that can be de- 
tected, and when variations of focus are taken into considera- 
tion, which with so low a power is a very important factor, I 
should be inclined to place the limit considerably higher than 
the lower figures above quoted. With very high powers on the 
other hand very much better work can be done with the glass 
eye-piece micrometer as will be apparent from the above tables. 
Yet even here, owing to the fact that the edges of lines ruled 
on glass or metal, or the outlines of blood corpuscles are not so 
sharply defined as with the lower powers. I am inclined to 
think that with such a micrometer it would be doing extremely 
well to be sure of accuracy at the eye end of the instrument to 
within one whole division. 
The value of one division of the filar micrometer above de- 
scribed with the above objectives is as follows : 
mae 2e1ss, 1 div. —--..-—. ~- -<22eeee =n 4923 pu. 
eee ee he) eee 329 pw. 
1-4 ‘* “) Xeev.-cor..10°) 1 digiee-- 2 = 096 
1-4 ** Bausch & Lomb (opaque illuminator) 
(CN eS eee RT 126 yp 
H-Gaineh Zeiss (cov. cor. 15°).--2eee se 0684p. 
Peter OPCMCCh = ee 04 
1-10 - ‘* ‘* with Bausch & Lombamplifier .02554 yu. 
EA eee ee M217 ys. 
As I before remarked the filar micrometer is more perfect 
than the eye; so that as every one knows it is by no means 
possible to make all or any considerable number of a series of 
