36 THE Microscope. 
either side. This will account for the want of unformity in 
some of the series measured with the glass eye-piece microme- 
ter. With the lowest power the series are uniform, for the rea- 
son that the difference could not be measured with so coarse an 
instrument. It would in the latter case be like attempting to 
measure a space of z/s0 inch with an ordinary carpenter’s rule. 
With respect to the last two tables, the figures in the second 
decimal place indicating hundredths of a division, should prop- 
erly have been omitted, and of course cannot be relied upon; 
but to avoid the labor of recasting the table I have inserted 
them with this explanation to prevent any one’s being misled 
thereby. 
From the above measurements I think it clearly appears 
that with low powers the glass eye-pice micrometer as com- 
pared with the filar micrometer, is a decidedly inferior instru- 
ment; but when used with a very high power so that each di- 
vision represents only a small fraction of one mikron, it becomes 
in the hands of a skillful observer a very good instrument. 
It may perhaps be objected to the above measurements 
that the disagreement between the different series made with a 
filar micrometer is an argument against its accuracy. To this 
objection I would answer that I am not in this investigation at- 
tempting to settle the absolute length of the respective inter- 
vals, but only to show the sensitiveness, so to speak, of the filar 
micrometer as compared with the ordinary eye-piece microme- 
ter. This objection might be obviated by making the measure- 
ments more numerous, while no number of measurements, how- 
ever great, will eliminate the accidental errors of an instrument 
so coarse as respects the spaces measured that the very small dif- 
ference in the lengths compared cannot be detected by it, as is the 
case with the ordinary micrometer when used to compare 
lengths, which with the power used appear to be equal. The filar 
micrometer when well made is more accurate than the eye and 
with it a careful observer can by making a series of measure- 
ments easily detect differences of length which can not be de- 
tected by a careful observer with any eye-piece micrometer of 
the ordinary pattern however well made. Repeated trials make 
me quite confident of the truth of this statement ; and in order 
to further test the accuracy of this conclusion by reference to 
