A WAVE-LENGTH COMPARATOK FOR STANDARDS OF LENGTH. 21 



the movable carriage), and n = the number of supports, for the points of support of a 

 bar or elongated plate without flexure. This movement is a particularly steady and 

 easy one, very superior to the transverse movements hitherto given to comparators. 



The upper table, v, rests with only f inch interval on the transversely movable 

 carriage, Q, by means of three levelling screws, two near the left end and one near the 

 right end, also at the Airy positions ; their heads, B,, are large milled flat discs 

 projecting outside the width of the carriage just sufficiently to enable the observer 

 to manipulate them easily. The single levelling screw on the right rests in a short 

 longitudinal V-groove in a gunmetal slider let transversely into the intermediate 

 carriage top, the transverse sliding motion of which can be brought about by means 

 of a large milled-headed screw in front facing the observer. The levelling screw is 

 thus carried with it, the V-slot providing for the slight longitudinal component of the 

 motion which occurs. Of the two levelling screws at the left end the back one is 

 bluntly pointed and rests in a conical depression in the intermediate carriage top, 

 and forms the pivot of the whole operation of this slight adjustment for azimuth. 

 The front left screw has a slightly spherical but almost flat termination, and merely 

 rides on the surface of the intermediate carriage during the operation. These three 

 levelling screws thus provide at the same time the means of levelling, slight raising 

 or lowering, and, with the aid of the milled-headed front screw, for slight azimuth 

 adjustment of the bars. They afford, in fact, all the adjustments required for insuring 

 parallelism of the defining marks on the standard bars and the micrometer spider- 

 lines, and for finely adjusting a defining line to the focus of the microscope, when it is 

 not desired to touch the microscope itself for fear of disturbing its adjustment as a 

 beam compass, in comparing a second bar with a standard bar. 



The ordinary table-top, for all experimental purposes, is simply a truly planed 

 surface on which bars, Grayson rulings, or other objects under observation can be 

 directly supported. The second of the three interchangeable table tops is the one 

 which is shown marked ?', in figs. 3 and 4. It is specially constructed to support 

 a pair of standard bars independently, each on a pair of friction rollers at 

 the Airy positions, or at any positions officially required. The two bars are thus 

 separately supported in order that, whilst the back one is fully adjusted by the means 

 provided with the intermediate carriage and the three large levelling screws which 

 this table top carries precisely like the first one, the second bar may in addition be 

 independently adjusted for azimuth, level, and longitudinal position, by means 

 provided in the mode of fitting its pair of rollers. This enables two bars of slightly 

 different dimensions, say, as regards shape and size of section and depth of defining- 

 line wells, to be brought so that their four defining lines, two on each bar, are in 

 precisely the same plane. This adjustment also greatly facilitates work with Grayson 

 rulings and high powers of the microscopes. Moreover, exact parallelism of the bars 

 can be obtained, and their defining lines at any rate those at one end if there are 

 appreciable differences between the bars can be brought exactly opposite to one 



