PHYSICS: A. A. MICHELSON 397 
This theoretical Hmit can be reached, however, only on the condition 
of an extraordinary degree of accuracy in the spacing of the lines. 
Several methods for securing this degree of accuracy have been attempted 
but none has proved as effective as the screw. This must be of uni- 
form pitch throughout and the periodic errors must be extremely small. 
For a short screw for example, one sufhcient for a grating two inches 
in length, the problem is not very difhcult but as the length of the 
screw increases the difhculty increases in much more rapid proportion. 
It was solved by Rowland in something over two years. 
Since this time many problems have arisen which demand a higher 
resolving power than even these gratings could furnish. Among these 
is the resolution of doubles and groups of lines whose complexity was 
unsuspected until revealed by the interferometer and amply verified 
by subsequent observations by the echelon and other methods. 
Others that may be mentioned in this connection are the study of 
the distribution of intensities within the spectral 'lines'; their broaden- 
ing and displacement with temperature and pressure ; the effect of 
magnetic and electric fields, and the measurement of motions in the 
line of sight, as revealed by corresponding displacement of the spectral 
lines in consequence of the Doppler effect. 
All of these have been attacked with considerable success by obser- 
vations with the echelon, the interferometer and the plane-parallel 
plate. These methods have a very high resolving power, but labor 
under the serious disadvantage that adjacent succeeding spectra over- 
lap making it difficult to interpret the results with certainty. 
Some twelve years ago the construction of a ruling engine was under- 
taken 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 ruHng. 
The principal claim to novelty of treatment of the problem lies in 
the application of interference method to the measurement and cor- 
rection 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 ad- 
justment 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 
