A WAVE-LENGTH COMPARATOR FOR STANDARDS OF LENGTH. 19 



the slider a tempered steel ring is rigidly fixed, having on the top side a strong 

 rectangular projection with slightly spherical surfaces. This projection engages and 

 brings about the motion of the microscope-carrying inch-thick bed-plate n, the 

 all-important cast-iron slider on the steel block (by V-and-plane motion) which carries 

 the microscope MI or M 2 and its counterpoise m^ or m 2 , and whose fine motion it is 

 the object of the fine screw to bring about. Of all the various movements which the 

 author has tested, for bringing about the movement of the movable glass plate of the 

 two such plates forming the interference apparatus, that of a solid cylinder sliding 

 fairly tightly without rotation in a cylindrical bore the fitting of the two being very 

 accurate and the length of both adequate to minimise any minute amount of residual 

 play has always proved the most satisfactory, where a V-and-plane could not be 

 used. In this case both are used, the cylindrical motion to push the block carrying 

 the microscope and glass plate along, and the V-and-plane as the type of motion of 

 that block itself. Hence a particularly steady movement is employed to bring about 

 by pushing the absolutely steady movement desired. The object is wonderfully well 

 achieved by this arrangement. The projection enters a suitable steel socket with a 

 rectangular recess let in to the underside of the inch slider, and projection and socket 

 are so accurately fitted as to need no further provision for an absolutely free and 

 steady motion. 



To facilitate the free motion of this microscope-carrying slider, and to reduce the 

 driving strain on the fine screw, however, it is necessary to relieve some of the dead 

 weight of the microscope and its bracket, cr, and counterpoise ; this has been 

 accomplished in a highly satisfactory manner by four spring-pistons let into the lower 

 steel block-slider o, the ends of which are fitted with steel rollers. These spring-pistons 

 are so disposed as to relieve the weight to the extent of about two-thirds of the total 

 weight of the inch slider and all that it carries. The two springs at the microscope 

 end of the slider, where the main weight occurs, are considerably more powerful than 

 the other two, the relations being such as to provide for equal facility of motion 

 throughout the whole length of the slider. 



This device is found to quite successfully relieve the 50-thread screw from undue 

 strain, and enables an absolutely free and surprisingly steady motion to be given to 

 the microscope. The proof of this is the perfectly steady motion of the interference 

 bands, when the endless screw is rotated, either by hand by means of the milled 

 head or by the large wheel and its attached shaft. The success of this operation, 

 which carries with it the success of the whole instrument, is largely due to the 

 admirable manner in which Messrs. Trough ton and Simms have constructed this part 

 of the apparatus. 



The Standard-bar Carriage. 



A few details concerning the mode in which the ba,r carriage is moved, so as to 

 bring the bars under the microscopes, involving either their longitudinal or their 



D 2 



