238 SUMMARY OF CURRENT RESEARCHES RELATING TO 



motion adequate to bring either of the two bars to be compared under 

 the Microscopes, as well as fine-adjustments for azimuth, height, and 

 level, thus enabling the defining marks on the bars to be readily focused 

 without touching the Microscopes if it is so desired. 



Each Microscope is carried on a solidly constructed slider on the 

 Y-and-plane bed, by which its coarse-adjustment for position is effected. 

 The microscope-bearing bracket is not, however, fixed directly to this 

 slider, but to a second one sliding over the first, also with V-and-plane 

 contact, and with the further control of the movement of a cylinder 

 within a cylindrical boring. The fine-sliding is effected by means of a 

 most carefully made screw of fifty threads to the inch, on which the 

 success of the instrument depends, and which carries at its outer end a 

 large milled head for hand rotation, and a worm-wheel of 100 teeth 

 gearing with an endless screw, which can either be rotated by hand by 

 means of a milled head or by means of a shaft and a large wheel, seen 

 in front in the illustration. One complete rotation of the latter corre- 

 sponds to the movement of the Microscope and the black glass 

 interference disk to an extent which causes the passage of fifteen 

 interference bands past the reference centre. More than an inch of 

 movement of the circumference of the wheel is necessary to effect the 

 passage of a single band. Two-thirds of the dead-weight of the 

 Microscope and slider are taken up by four spring pistons, and the move- 

 ment of the slider by the screw is only a push in either direction against 

 the walls of a recess in the free slider, there being absolutely no strain 

 anywhere. Hence this movement of the Microscope is not only an 

 excessively fine one, but also so steady that the bands pass with a pre- 

 cision which leaves nothing to be desired, and each band may be held 

 for any length of time for counting purposes. 



Each Microscope is provided with a micrometer eye-piece, with 

 spider-lines arranged as in the interferometer. The fine-adjustment is 

 made exceptionally steady and regular. Two sets of objectives are 

 provided, one pair for observing the defining lines in the countersunk 

 wells near the ends of standard bars, with a magnification of 150 

 diameters, and without penetration of the well by the objective, and the 

 other set for use with the wave-length rulings. 



The defining lines, of whatever character, are illuminated (with 

 " critical illumination ") by the brilliant image of a distant Nernst 

 lamp, with the aid in each case of a little reflecting prism, a collimating 

 lens, an iris diaphragm, and a glass-plate mirror above the objective, all 

 provided with fine-adjustments. This avoids all heating effect on the 

 bars, and the last traces of heat rays are filtered out by a thick water- 

 jacket in front of the lamp and its beam-parallelising lenses. The 

 illumination of the wave-length rulings ^^hnr m - apart is excellent with 

 the -fa m - dry objectives employed, and the definition truly surprising. 



The temperature of the whole comparator room is maintained at the 

 official temperature, 62° F., entirely electrically, both as regards artificial 

 heating and the thermostat, which is original. So sensitive is the latter 

 that the entrance of a person into the room is immediately followed by 

 the extinction of one of the heating lamps to compensate for the extra 

 w T armth introduced. 



The finest defining lines yet employed on any line-measure bars are 



