1915-] OF A TEN-INCH DIFFRACTION GRATING. 139 



spectra overlap, making it difficult to interpret the results with 

 certainty. 



Some twelve years ago the construction of a ruling engine was 

 vtndertaken with the hope of ruling gratings of fourteen inches — 

 for which a screw of something over twenty inches is necessary. 

 This screw was cut in a specially corrected lathe so that the original 

 errors were not very large, and these were reduced by long attrition 

 with very fine material until it was judged that the residual errors 

 were sufficiently small to be automatically corrected during the 

 process of ruling. 



The principal claim to novelty of treatment of the problem lies 

 in the application of interference method to the measurement and 

 correction of these residual errors. 



For this purpose one of the interferometer mirrors is fixed to 

 the grating carriage, while a standard, consisting of two mirrors at a 

 fixed distance apart, is attached to an auxiliary carriage. When 

 the adjustment is correct for the front surface of the standard, 

 interference fringes appear. The grating carriage is now moved 

 through the length of the standard (one tenth of a millimeter if the 

 periodic error is to be investigated; ten or more millimeters if the 

 error of run is to be determined) when the interference fringes 

 appear on the rear surface. This operation is repeated, the differ- 

 ence from exact coincidence of the central (achromatic) fringe 

 with a fiducial mark being measured at each step in tenths of a 

 fringe (twentieths of a light- wave). As a whole fringe corresponds 

 to one hundred thousandth of an inch, the measurement is correct to 

 within a millionth of an 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 completely eliminated and the resulting gratings have 

 very nearly realized their theoretical efficiency. 



A number of minor points may be mentioned which have con- 

 tributed 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 



