C. Barus — The Comparison of Two Screws. 335 



also coincide horizontally in the field, i. e., the two images of 

 any specks of dust or cross lines in the slit should cover each 

 other. If this is not the case the tablets carrying M and JV 

 must be provided with three adjustment screws, so that the 

 condition of mirror normal to the beams of light may be 

 reestablished without changing a and a'. In other words, the 

 adjustment screws of the mirrors on the micrometer must be 

 left untouched. 



When this is done, the ellipses will be seen in the direction 

 of the refracted rays R as soon as the proper distances m and n 

 have also been reestablished, by moving either mirror alone. 

 The arrangement of mirrors on the tablet should therefore be 

 as nearly as possible symmetric, even when different forms of 

 screws are compared. 



A simple type of micrometer screw or attachment suitable 

 to any screw to be tested will also be used below and may be 

 described here. S, fig. 2, is the micrometer screw in question, 

 with its graduated head at i^ revolvable in the socket R, this 

 being adjustable, with three screws as usual, on the tablet at 

 either JV or M, fig. 1. The mirror M, fig. 2, is separately 

 adjustable with three screws (horizontal and vertical axes), s, s, 

 and a strong spring suggested at p. The whole arrangement 

 Mps is firmly fixed to the end of the screw S. In this case 

 the mirror rotates with the screw as it advances and the latter 

 is therefore necessarily normal to the component beam m, an 

 adjustment secured by the set screws at s in the usual way, 

 the effect of the operations being observed in the telescope T 

 in fig. 1. The approximate adjustment is conveniently made 

 with sunlight returned by reflection from the mirror to a screen 

 20 or 30 feet distant. Suppose, fig. 3, the mirror is nearly 

 normal to the screw ; let the intersection of the incident ray, 

 the screw axis, the normals, and the reflected rays with the 

 screen, all lines prolonged if necessary, be at L, S, JV' and R, 

 respectively. If the screw is rotated 360°, the normal will 

 trace a circle JV' to JV IV , the reflected ray a similar circle R' 

 to R 1V . If the normal JV' coincides with the axis of the screw 

 S, the reflected ray will be at the center, R, of the circle R'R". 

 Hence the center is to be sought by rotating the screw. The 

 mirror is now adjusted so that the reflected ray is at R. Then 

 JV is at S, as required. With successive trials this succeeds 

 very well. 



3. Data. — A trial comparison was made of the screws of two 

 slide micrometers, one of them old but with an excellent slide, 

 the other with a good new screw but an imperfect slide. As 

 a result of this it was impossible to keep the ellipses adequately 

 sharp throughout the whole motion of the screw. In fact, it was 

 necessary to make a readjustment of mirrors M and JV from 



