1878. | Microscopy. | 837 
micrometers, though not to be compared with the work of Nobert 
or of Rogers. The screw was then abandoned, and the machine 
prepared with which the present work is done. Unfortunately, 
the maker is unwilling to give further information about this ma- 
chine than that it has other than a screw motion, intimating, per- 
haps, that the spacing is accomplished by a lever movement. The 
finest bands are ostensibly ruled to ygqyoq inch,’ but it may be 
considered doubtful whether they are ruled according to the read- 
ing of the machine that made them. The bands up to gg$ap inch 
have been reported correct by authority that ought to be com- 
petent to judge, and the whole series is ih ae of.careful study 
those who are expert in such work. O very different 
class is the progress made by Professor Rogers paie the past 
metry have scarcely claimed to possess a definite degree of ac- 
curacy. The speaker had used for years a standard obtained by 
comparing about two hundred micrometers ruled in different parts 
of the world, rejecting the few that from their p — wee 
the average were presumably erroneous, and averaging the re- 
mainder. “Probably such a standard, though cease Ceca 
of uncertain value, was as nearly safe as anything heretofore at- 
tainable. To obviate this uncertainty, Professor Rogers determ- 
ined to derive a standard inch, with a precision not known to have 
been obtained before, from “ Bronze bar No. 11,” which has been 
the authorized standard in this country since 1855, having been 
presented to the United States by the British Government as one 
of the five original duplicates of “ Bronze 19” the only English 
standard since the “ standard yard of 1760” was destroyed by 
fire, and to jiasi a standard centimetre from the iron bar now in 
possession of the United States Coast Survey, one of the original 
standard metres, and the best representative in this country of the 
platinum metre deposited in the Archives of Paris. He fitted a 
glass bar forty-two inches long, one inch wide and one and a 
quarter inches thick, into an iron frame, the surfaces being made 
as true a plane as possible. With a diamond in a a graduating en- 
gine he laid off upon it one hundred centimetre divisions. 
then took the bar to Washington where the said lines were com- 
pared with the standard metre, and where the British yard and the 
Archives metre were transferred from the Saxton comparator of 
the United States Coast Survey to both the glass bar and its iron 
frame. The errors of the odaia of the yard, and of the 
metre, were investigated by means of a comparator constructed 
for the purpose of subdividing any unit into equal parts. The cor- 
rections for each subdivision having been determined, a number 
Fasoldt’s finest lines arè now made in serra par the lines of the 
_ lowest band being 1-10,000 of an inch a, of the next band 1-20,000 inch, of the 
so on, increasin ng 10,000 to t ee each time to the twelfth. 
, has lines spaced. D I-120,000 inch. These plates 
made at as low a price as $15.00 
