THE MICROSCOPE, 7d 
lines first downward and then upward, thus introducing unknown 
errors due to the causes above specified, would deprive the results of 
any value they might otherwise possess. There is no means of inter- 
comparison and of eliminating these unknown errors. 
I cannot ascertain, however, from the paper, with what standard 
the ;4; inch was compared, or exactly how it was compared. If, as 
I suppose, it was compared with the screw of a screw stage microme- 
ter, which was assumed to be a constant, I must beg to dissent from 
any conclusion thus obtained. I find it necessary, in ruling 
standards of any considerable length, to assume a value for the 
screw, rule a trial scale, and by actual comparison with some 
authentic standard deduce therefrom a series of corrections 
before ruling the final scale. If great accuracy is desired, it 
may be necessary to repeat this several times before ruling the final 
scale ; and this is the case notwithstanding the errors of the screw 
have previously been carefully investigated. I would never trust 
any screw or train of wheels as a final standard of reference for 
more than about one-half the field of the microscope, much less for 
so long a space as ;4; inch. 
I do not think therefore that the results of the measurements 
of Mr. Fasoldt are entitled to any weight whatever, as invalidating 
the final correction of ‘“‘A” and its so-called copies, if they were 
intended for that purpose ; and indeed I cannot see that they have 
any scientific value whatever, in the absence of more precise inform- 
ation as to the conditions under which they were made. Under 
precisely identical conditions, so far as they can be commanded, - 
some slight variation between different single measurements is to be 
expected; but the substantial agreement existing between the results 
arrived at by independent observers, at widely different times and 
places, is not to be invalidated by discordant peau obtained by one 
observer under different conditions. 
Centimeter scale “A” was investigated by all the observers 
under substantially the same conditions; and the copies by Mr. 
Fasoldt were necessarily compared with it under similar conditions, 
viz: by the use of a Bausch & Lomb opaque illuminating objective, 
so that if any errors were introduced by the fact that the lines of 
Mr. Fasoldt’s copies are on the under side of the cover-glass upon 
which they were ruled, these errors are constant and are the same as 
if the scales in question were ruled with these errors incorporated in 
them. It should be remembered always that “‘A” is uncovered, and 
that the Fasoldt copies are permanently mounted, the lines being on 
