242 
FISCHER. 
The Committee metre is an end standard, and prior to 1890 
it was the metric standard of the United States Coast and 
Geodetic Survey and of the Office of Standard Weights and 
Measures. The Repsold metre is a line standard, and as it 
had been compared at the International Bureau in 1883 the 
relation of the Committee metre to the International metre 
was established by Mr. Tittmann’s observations. 
While the method of Fizeau * is known to every metrol- 
ogist, a brief description of it here may not be considered 
out of place. The method depends upon the fact that the 
image of an object reflected from a plane surface appears to 
be as far behind the surface as the object is in front. This 
is taken advantage of by bringing opaque points or a spider 
thread nearly into contact with each of the end surfaces of the 
bar to be compared, thus making it possible to observe on 
the end surfaces by estimating the centre of the space be¬ 
tween the points or thread and their reflected images. The 
distance between the end surfaces, as determined in this way, 
may then be compared with a line standard on any compar¬ 
ator suitable for comparing line measures. 
It had been pointed out by Fizeau that if the focusing 
upon the points and their reflected images be imperfectly 
done, errors would result from the fact that only one-half of 
each objective is used when the end standard is in position. 
This conclusion was based upon the following theoretical 
consideration of the lens : 
(1.) Let C (Fig. 4) be the aplanatic objective of a micro¬ 
scope and F the conjugate focus of the point 0. Assume, 
* Travaux et Memoires du Bureau International des Poids et Mesures, 
Tome X. 
