276 Mr. L. Bell on the Absolute 



value of Si and S2 can be in error by as much as one part in 

 half a million. 



So much for the standards of length. The comparator used 

 in the measurements was a very efficient instrument, particu- 

 larly suited for the purpose. It consisted essentially of a long 

 carriage (J metre) running on V-shaped ways and carrying 

 the microscope. This carriage slides against adjustable stops, 

 and is pressed against them with perfect uniformity by means 

 of weights. An adjustable platform below carries the stan- 

 dards and objects to be measured. The ways of both carriage 

 and platform had been ground till they were perfectly uniform 

 and true, and the working of the instrument left little to be 

 desired in the way of accuracy. Throughout a long series of 

 measurements the stops would not be displaced by so much 

 as O'lfi if proper care were used in moving the carriage. 

 The microscope was attached so firmly as to avoid all shaking, 

 and was armed with a half-inch objective and an excellent 

 eyepiece-micrometer. The objective was made specially for 

 micrometric work, and was fitted with a Tolles' opaque illu- 

 minator. Measurements were made as follows : — The standard 

 bar, and the grating mounted on a polished block of speculum 

 metal, were placed side by side — or sometimes end to end — 

 on the platform and very accurately levelled. The stops were 

 set very nearly three centimetres apart, one end of the grating 

 brought under the microscope resting against one of the steps, 

 and the micrometer set on the terminal line. Then the 

 carriage was brought against the other stop, and the micro- 

 meter again set. The same process was then gone through 

 on three centimetres of the standard, and then going back 

 to the grating it was compared in the same manner with 

 succeeding triple centimetres till the fifteen-centimetre line 

 was reached, thus eliminating the errors of the single centi- 

 metres and making the determination rest only on the fifteen - 

 centimetre line. The temperature was given by a thermo- 

 meter placed against the standard bar or the block that carried 

 the grating. In this manner each grating was repeatedly 

 compared with the first fifteen centimetres of each bar, at or 

 near 20°, the temperature at which the gratings had been 

 used. The micrometer constant was determined by measuring 

 tenths of millimetres ruled on Prof. Rowland's engine; but in 

 practice the stops were so adjusted that it was almost elimi- 

 nated. Each division of the micrometer-head equalled 

 0'28/ul, and the probable error of setting was less than half 

 that amount. 



All measurements were reduced to 20° C, as in the case of 

 the angular determinations. The line along which the linear 

 measures were made was that formed by the terminations of 



