ON THE COMPARISON OF LINE AND END STANDARDS. 245 
able that part of the discrepancy in the two results is due to 
the observers having adopted different standards in focusing. 
The difficulty in determining when the points and their 
images were in focus w'as increased by the poor reflecting 
surfaces of the Committee metre, which rendered the reflected 
images much less perfect than the points themselves. 
The difficulty of providing the comparator used in the 
later observations with the necessary adjustments to render 
the use of the movable screen possible, and also the inferior 
surfaces of the Committee metre, led to the consideration of 
other methods. 
Several special contact methods for comparing line and 
end standards, notably that of Airy,* had been suggested and 
used, but none of them seemed applicable to this case. 
Fig. 5. 
Auxiliary abutting pieces have been used for many years 
in the Office of Weights and Measures to determine the dis¬ 
tance between the interior surfaces of the matrices (Fig. 5) of 
the State yards t on the Saxton Comparing Machine. J 
The manner of using these pieces is exceedingly simple. 
The abutting ends are first placed in contact with surfaces of 
the matrix, as shown in Fig. 5, and the distance between the 
lines ruled upon them is compared with the distance between 
0 and the 30th-inch stop of the Saxton Comparing Machine. 
The abutting ends are then placed in contact with one another 
and the distance between the lines is compared with the space 
between the 30th to 36th inch stop. The result of these opera¬ 
tions is that the length of the matrix is referred to the dis- 
* Philo. Trans, of the Roy. Soc. of London, vol. 147, part iii, p. 685. 
f Standard yards made for the States. 
t Report of the Secretary of the Treasury on the Construction and Dis¬ 
tribution of Weights and Measures, Wash., D. C., 1857. 
