124 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



edges of the slit separate equally from the unchanged middle of the 

 slit. 



The movement of the screw is read off on a divided scale T fixed to 

 the axis, of which 50 divisions, about 1 • 57 mm. apart, correspond to 100th 

 of a millimetre. The scale readings were controlled by placing the slit 

 apparatus on the stage of the Microscope and measuring the width of the 

 slit with the eye-piece micrometer under a magnification of 500. 



Fig. 16. 



On loosening the screw M which fixes the scale to the axis, the former 

 can be turned about a so as to bring the zero point into position. 



The piece of white card p seen in fig. 16 is used for the better 

 illumination of the scale. 



The mechanism of the second slit, of which only the scale T' and 

 screw-head M' are seen in the fig., is exactly similar. In order, in 

 case of accidental displacement of the edges, to bring the middle of the 

 one slit exactly in the line of prolongation of the other, and so insure 

 accurate superposition of corresponding parts of the two spectra, the 

 two edges are fastened by means of two adjusting screws on the plates 

 / and /' carried by the blocks e and e'. 



The upper piece, the details of which are shown in vertical section 

 in fig. 14, consists of the box A' containing the prism system P, which is 

 composed of two prisms of crown glass (refractive index for the yellow 

 rays 1-611, refractive angle 40° 20') and one of flint (index 1-691, 

 angle llO'^ 42'). Beneath the box A' is screwed the collimator-tube a', 



