398 
PHYSICS: A. A. MICHELSON 
investigated; ten or more millimeters if the error of run is to be de- 
termined) when the interference fringes appear on the rear surface. 
This operation is repeated, the difference 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 measure- 
ment 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 al- 
most completely eliminated and the resulting gratings have very 
nearly realized their theoretical efficiency. 
Enlargement of photograph of the green mercury 'line' X5461 taken by H. E. Lemon with 
10-inch diffraction grating in sixth order. Scale: 1 division = 0.01 A. U. Ruled surface, 
9f X 2| inches, 11700 lines per inch. Mounted in Littrow form with 8-inch lens by 
Brashear. Focal length, 20 feet. 
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 true; and these 
were straightened by application of an auto-collimating device which 
made the deviation from a straight line less than a second of arc. 
Q}) 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 
twenty to forty pounds) by floating on mercury. 
(c) The longitudinal motion of the screw was prevented by allow- 
ing its spherically rounded end to rest against an optically plane surface 
of diamond which could be adjusted normal to the axis of the screw. 
