LI. Bell—Absolute Wavelength of Light. 171 
are both exquisite specimens of the work which Prof. Rowland’s 
engine is capable of doing, though as the event showed, I is 
decidedly the better grating, in the matter of regularity of ruling. 
ANGULAR MEASUREMENTS. 
At the beginning of the work a serious question of adjustment 
arose. There aretwo ways of using a grating perpendicular to 
one of the telescopes. In the first place it may be placed and 
kept accurately in that position, and secondly it may be placed 
nearly in the position for normal incidence, and the error meas- 
ured and corrected for. Angstrém used the latter method, which 
involved a measurement on the direct image of the slit as well 
as on the linesobserved. Using Angstrdém’s notation—let a and 
a’ be the readings on the spectra, and M that on the slit. Let, 
also, 
ata’ a—a’ 
5 —M=aA and 5 ==) 
Then if 7 is the angle made by the incident ray with the nor- 
mal to the grating, and N the order of spectrum, 
Nd : 
a ae OE (vy+4) sin pg; 
also sin y = sin (y+4) cos g, 
COS @ : 
and tan y = —— A. But from the second of 
1—cos@ 
the above equations, 
d sin V 
DAS) Sore 
sin (v+A) ee 
Now it was found that with the collimating eyepiece belonging 
to the spectrometer, 7 would never exceed and seldom reach 
10”, while the angles of deviation observed were about 45°. 
Substituting these values in the last equation, it at once 
appeared that the cosine of (y+ 4) was aquantity differing from 
unity by considerably less than one part ina million, and hence 
entirely negligible. Further, it was found that the grating once 
set could be trusted to remain perpendicular through a series of | 
measurements, and though at the end of each series the grating 
was adjusted to a new part of the circle, and a close watch was 
kept for its slipping out of adjustment, it was never found 
necessary to reject a series from that cause. 
The grating was centered and adjusted with reference to the 
circles and their axes by the ordinary methods. Throughout 
the experiments the light was concentrated on the slit by an 
achromatic lens of about half a meter focus, which was placed 
behind a sheet of deep yellow glass, which served to cut off the 
