40 Tue Microscope. 
measurements exactly alike, yet I think I speak within bounds 
when I say that with low powers it is, at the very least, 
ten times as accurate as the eye-piece micrometer, and when 
the mean of a series is taken, there is in my mind no doubt 
whatever that itis much more reliable. I think it would not be 
difficult to demonstrate this. I had intended to apply the 
method of least squares to the elucidation of this question, but 
so far as the limits of accuracy of the microscope are concerned, 
the work has already been done much better than I can do it by 
Prof. Rogers and Prof. Morley in a paper upon the subject pub- 
lished some years since in the (then) American Quarterly 
Journal of Microscopy. Moreover, while the results obtained 
by the filar micrometer with any power might be tested by this 
method, the results obtained by its application, for instance, to 
the comparison of the 2d and 3d m. m., of Scale A, with the glass 
eye-piece micrometer above referred to, would be fallacious in the 
extreme; for as every measurement with the glass eye-piece mi- 
crometer with this power would be absolutely identical there 
would be no residuals and no probable error. Notwithstanding 
this, there might be an error in any measurement of 2 or 3 mi- 
krons which this instrument would not make apparent. When 
a power is used sufficiently high to disclose the difference of 
Jength this objection would not apply. 
The conclusion of the whole matter, therefore, is that for 
the comparison of lengths nearly equal and for the measurement 
of minute distances with low powers, the glass eye-piece mi- 
crometer is vastly inferior to the filar micrometer; and that in 
cases where the greatest attainable accuracy is required, as for 
example in the measurement of blood corpuscles in criminal 
cases nothing but the filar micrometer should be used. I shall 
have something to say about the much inferior method of meas- 
urement by means of the camera lucida at some future time. 
—EEEEEE, 
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