34 THe MICROSCOPE. 
1882,’ to which I have recently had access. The above remarks 
apply to means of series of measurements of several different 
spaces, made with a filar micrometer with objectives ranging 
from one inch to 3; inch. I would not be understood as saying 
that these measurements are accurate to the extreme iimit in- 
dicated by the above figures, but only that the values of one 
division deduced from the different series agree to that extent. 
The extreme limit of accuracy in microscopic measurement is a 
subject upon which I am not yet ready to express a decided 
opinion. 
Coming now to the particular tests used in testing my filar 
and ordinary eye-piece micrometer ruled on glass to 7.5, inch 
with every fifth and tenth line longer than the others, I took as 
spaces for comparison two spaces nominally of zoo inch on each 
of a Beck’s and Fasold’s glass stage micrometers, and 10 spaces 
of z++ mm. on a standard centimeter ruled on speculum metal 
by Prof, W. A. Rogers, and made a series of from five to fifteen 
measurements of each space with a filar and an ordinary eye- 
piece micrometer, took an arithmetical mean and compared 
the results using a Zeiss 1”, 3-5” and }” objective, as follows: 
OBJECTIVE. | oe EO RECE {ier 1-100 BECK) "BEd REMARKS. 
MIKRONS. MIKRONS. 
C C G4") Zeiss) Filar 25.29 ; 25.45 i Means of 6 measurements. 
rr vs ‘© | Filar 25. 41 25.66 Means of 4 measurements, 
Means: 25.35 25.55 Difference .20m. 
“ ss ‘* |Glass 1-1000 inch 25.04 25.09 ‘ Means of 5 measurements. 
es a ‘* /Glass 1-1000 inch} 25.48 i 25.48 Means of 4 measurements. 
Means: 25.26 25. 28 Difference .02 m. 
A A (8-5) Zeiss! Filar 24.99 25.15 Means of 5 measurements, 
an “ ++ | Filar 25.01 25.19 Means of 4 measurements, 
a0 + |Rilar 25.02 25.25 Means of 5 measurements. 
Means: 25.01 25.20 Difference .19 m. 
“ On ** |Glass 1-1000inch 25.73 26.17 Means of 6 measurements. 
“ on ** |Glass 1-1000inch 29.15 25.15 Means of 4 measurements. 
an, pede ** |Glass 1-1000 inch 24.08 25.47 Means of 6 measurements. 
Means: 24.99 25.26 Difference .27 m. 
a a (1) Zeiss\Filar 25.14 29. 29 Means of 10 measurements. 
Difference 15 m. 
as, has ** |@lass 1-1000 inch 25.00 25.00 Means of 5 measurements, all alike. 
Difference 000. 
jth 1-1000 
2d 1-1000 FASOLD} ‘pasoLp 
aa (1”) Zeiss|Filar 25, 16 25.49 Means of 6 measurements, 
Difference .33 m. 
ast yaues ** |@lass 1-1000inch 25.00 25.00 Means of 8 measurements, all alike. 
Difference 00. 
A A (8-5”) Zeiss|Filar 25.26 25.53 Means of 6 measurements. 
Difference .27 m. 
osy | abe *« |Glags 1-1000 inch 25.20 25, 20 Means of 5 measurements, all alike. 
Difference 00. 
