PHYSICS: A. A. MICHELSON 
399 
(d) The screw was turned by a worm wheel (instead of pawl and 
ratchet) which permits a simple and effective correction of the periodic 
errors of the screw throughout its whole length. 
(e) A correcting device which eliminates periodic errors of higher 
orders. 
(/■) It may be added that the nut which actuates the carriage had 
bearing surfaces of soft metal (tin) instead of wood, as in preceding 
machines. It was not found necessary to unclamp the nut in bringing 
it back to the starting point. 
Finally it may be noted that instead of attempting to eKminate the 
errors of the screw — by long continued grinding which inevitably leads 
to a rounding of the threads — it has been the main object to make these 
errors conveniently small; but especially to make them constant — for 
on this constancy depends the possibility of automatic correction. 
The accompanying photograph made with a 10-inch grating sixth 
order, (actual ruled surface 9.4 inches by 28 inches), used in the Littrow 
form with an excellent 8-inch lens by Brashear, is given in evidence of 
its performance. The resolving power as shown by the accompanying 
scale of angstrom units is about 450,000. The original negative shows 
a resolving power of about 600,000. The theoretical value is about 
660,000. 
Doubtless the possibility of ruHng a perfect grating by means of the 
light-waves of a homogeneous source has occurred to many — and 
indeed this was one of the methods first attempted. 
It may still prove entirely feasible — and is still held in reserve if seri- 
ous diJQ&culty is encountered in an attempt now in progress to produce 
gratings of twenty inches or more. Such a method may be made partly 
or perhaps completely automatic, and would be independent of screws 
or other instrumental appliances. 
It may be pointed out that an even simpler and more direct appH- 
cation of light-waves from a homogeneous source is theoretically 
possible and perhaps experimentally reahzable. 
If a point source of such radiations sends its light-waves to a colli- 
mating lens and the resulting plane waves are reflected at normal incid- 
ence from a plane surface, stationary waves will be set up as in the 
Lippman plates; these will impress an inclined photographic plate 
with parallel lines as in the experiment of Wiener; and the only limit 
to the resolving power of the resulting grating is that which depends on 
the degree of homogeneity of the light used. As some of the constitu- 
ents of the radiations of mercury have been shown to be capable of 
