640 SUMMARY OF CURRENT RESEARCHES RELATDTQ TO 



securely held by clamping on the under side. These stops are each pro- 

 vided with a pair of electro-magnets, the poles of which do not come in 

 contact with the armature seen at either end of the Microscope plate. The 

 magnets are intended to overcome the unequal pressure due to ordinary 

 contact, a rack and pinion being used to move the plate. The magnets 

 are used to lock the Microscope plate at each end of its traverse between 

 the stops. 



Beyond the main base just described, and supported also on brick piers, 

 is an auxiliary cast-iron frame, which is provided with lateral and vertical 

 motion within limits of zero, and 8 in. and 10 in. respectively, for rough 

 or approximate adjustment, and upon the top of this frame are two car- 

 riages, which slide from end to end, a distance of about 40 in. Upon these 

 sliding carriages are placed tables provided with means of minute adjust- 

 ment, for motion lengthwise, sidewise, and for levelling, thus permitting 

 the adjustment of a standard yard bar quickly, and without the necessity of 

 its being touched with the hands after being placed upon the table imtil 

 the work of comparison is completed. 



The tubes of the Microscopes are 12 in. long and 1^ in- diameter with 

 eye-piece micrometers, and the objectives are fitted with Tolles's illumi- 

 nating prism just above the lower lens. 



This method of illumination has proved to be invaluable in the work 

 of comparing line measure standards, especially so in the case of bars 

 having lines ruled on polished gold surfaces at the bottom of wells sunk 

 one-half the depth of the bar. 



The first operation in the use of the comparator is to level the main 

 base ; then sliding the Microscope plate from end to end of the steel tubular 

 guides — having the Microscope adjusted so as to be in focus upon the 

 surface of the mercury held in a shallow trough, over which the Micro- 

 scope passes — the curvature due to flexure of the guides is determined, 

 and may be compensated for by counterweights at the various points of 

 support. 



In order to test this right-line path of the Microscope plate, the following 

 method is employed. A fine line is traced upon the plane surface of a 

 standard bar, extending throughout its entire length. This is accom- 

 plished by means of a cutting-tool attached to the Microscope carriage. 

 Then, reversing the position of the bar, a second line is traced near the 

 first, care being taken to have the distance between the two lines of 

 each end a constant quantity. If the distance between the lines is a con- 

 stant at every point, it is safe to assume that the horizontal curvature is 

 insensible. 



The extent of the effect of any horizontal curvature in the cylindrical 

 ways may also be found by comparing the lengths of two standards placed 

 at varying distances from the centre line between the ways. 



While the comparator has all the conveniences belonging to the ordinary 

 method of comparisons by means of two Microscopes, preference is given to 

 the " stop method." The adjustable " stop plates " are first set approxi- 

 mately at a distance apart equal to the lengths of the standards to be com- 

 pared. The Microscope plate having been brought into contact with the 

 left stop, the reading of the micrometer is made for coincidence with the 

 initial line of the standard. The carriage is then placed in contact with 

 the second stop and the reading for coincidence with the terminal line is 

 then taken. The bar to be compared now takes the place of the standard, 

 and micrometer readings are made as before. The difiierence between the 

 results of these micrometer readings gives the difference between the lengths 



