404 CARNEGIE INSTITUTION OF WASHINGTON. 



corresponds to 0.00001 inch, the measurement is correct to within 

 0.000001 inch. 



The corresponding correction for periodic errors is transferred to the 

 worm-wheel which turns the screw, and for errors of run to the nut 

 which moves the carriage. In this way the final errors have been 

 almost comj)letely eliminated and the resulting gratings have very 

 nearly realized their theoretical efficiency. 



A number of minor points may be mentioned which have contributed 

 to the success of the undertaking: 



(a) The ways which guide the grating carriage as well as those which 

 control the motion of the ruling diamond must be very tnie ; and these 

 were straightened by appUcation of an auto-collimating device which 

 made the deviation from a straight line less than a second of arc. 



(b) The friction of the grating carriage on the ways was diminished 

 to about one-tenth of that due to the weight (which may amount to 

 20 to 40 pounds) by floating on mercury. 



(c) The longitudinal motion of the screw was prevented by allowing 

 its spherically rounded end to rest against an optically plane surface 

 of diamond which could be adjusted normal to the axis of the screw. 



(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 eliminate 

 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. 



Doubtless the possibility of ruling 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 held in reserve if serious difficulty is encountered in an 

 attempt now in progress to produce gratings of 20 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 applica- 

 tion of light-waves from a homogeneous source is theoretically possible 

 and perhaps experimentally realizable. If a point source of such radia- 

 tions send its light-waves to a coUimating lens and the resulting plane- 

 Waves are reflected at normal incidence from a plane surface, stationary 



