250 
FISCHER. 
I regret that circumstances were such that further experi¬ 
ments with a bar having more perfect end surfaces could not 
be undertaken to fully test the method. I see no reason, 
however, to doubt that it is at least as reliable as the optical 
method even when used in connection with the perforated 
screen. Indeed the most recent experience of the Interna¬ 
tional Bureau of Weights and Measures * has been such as 
to almost warrant the cessation of the use of the optical or 
Fizeau method. It has been briefly this : Six end standards, 
five of which were constructed for the governments of Eng¬ 
land, Russia, Germany, Austria, and Bavaria, were compared 
with a line standard by the optical method. They were after¬ 
ward compared with the same standard by means of a con¬ 
tact method, the only detail of the method given being that 
the contact surfaces of the abutting pieces were rounded. 
The two sets of results, greatly to the surprise of the observers, 
differed on the average by 3.5 microns, or by one part in 
300,000, the contact method giving the larger results for the 
end standards. 'Suspecting that the objectives of the micro¬ 
scopes used in the first comparisons were responsible for the 
discrepancies because of imperfections, the first observations 
were repeated, the only difference being that the original 
microscopes were replaced by two others. The results of the 
last observations agreed perfectly with those obtained by the 
use of the contact method, and the conclusion was reached 
that the first results were erroneous. 
The relation of the present International metre to the old 
Metre of the Archives was determined by the Fizeau method, 
and the question naturally arises as to whether the assump¬ 
tion heretofore made that the bars are equal can now be ac¬ 
cepted. I am of the opinion that it cannot, but that a re¬ 
comparison will be necessary. 
* Comity International des Poids et Mesures, Proces-Verbaux des 
Seances de 1897, pp. 55-61, 
