L. Bell—Absolute Wavelength of Light. 169 
ean probably be obtained from the metallic gratings. For the 
reasons previously stated, this paper is confined to the results 
from glass ones. 
Now there are two quite distinct ways of using transmission 
gratings—first, perpendicular, or nearly so, to collimating or 
observing telescope ; and second, in the position of minimum 
deviation. The method in the first case is familiar—the proper- 
ties of the second are as follows : 
The general relation between the incident and the diffracted 
ray is: 
SN nee hie : mar 
sin 7 + sin (6 —-7) = ac 
When 7=o°, this gives the ordinary formula for normal inci- 
dence. Putting it in the form— 
PM) Se) Lie: 
Ne =, SD 5 cos (@—>5), 
the deviation represented by the angular term will evidently bea 
minimum when i=s and the wave-length will then be given by 
the formula 
2 Gee) 
N= Ae) sin =. 
m 2 
It is not easy to say which method of procedure is preferable, 
but on the whole the ordinary plan of normal incidence offers 
fewer experimental difficulties and therefore was adopted, par- 
ticularly as the spectrometer used was specially well suited to 
that method. It is quite certain that either method will with 
proper care give the angular deviation with a degree of exact- 
ness far surpassing that attainable in the measurement of the 
gratings. , 
THE SPECTROMETER. 
This was a large and solid instrument by Meyerstein with a 
circle on silver 32°" in diameter divided to tenths of a degree. 
This is read by two micrometer microscopes 180° apart. The 
pitch of the micrometer screws is such that one turn equals 
about 2’, and as the head is divided into sixty parts each of 
these represents 2’. The micrometer can, however, be set with 
certainty to less than half this amount. The collimating and 
observing telescopes are of 4™ clear aperture and 35°™ focal 
length and the lenses are well corrected. The collimator is 
fixed to the massive arms which carry the reading microscopes 
while the observing telescope is attached to a collar on the axis 
of the main circle and moves freely upon it or can be firmly 
clamped so as to move with the circle. The grating is carried 
