200 J- Stanley Rogers : 



Adjustments. 



The following are the essential requirements of a spectro- 

 meter of this type : The tube slit, the axis of rotation of the spec- 

 trometer and the axis of the film holder should all be parallel, and^ 

 in this case, vertical; the tube slit and the curved portion of the 

 film holder should be equidistant from the axis of rotation; the 

 ground face of the crystal should be so placed that the axis of 

 rotation lies in it ; the central ray from the ** focal spot " of the 

 target should pass through the centres of the slits S^ and Sg,, 

 through the axis of rotation, and at right angles to it. 



After the vernier circle had been levelled, the axis of rotation 

 was found by so placing a vertical needle that its point, when, 

 viewed from above by a long focus microscope, remained sta- 

 tionary when the circle was rotated. The tube slit was then ad- 

 justed 10. cm. distant from the needle point, and was placed 

 vertical. The axis of the film holder was next made vertical by 

 means of the screws h (fig. la) ; and with the use of a centre 

 tester, it Avas brought into coincidence with the axis of rotation? 

 of the spectrometer. 



The crystal face was brought into the axis of rotation by so 

 placing it, that the position of the edge, as viewed in a low 

 power miscroscope, with its axis vertical, was in the same position, 

 both before and after the crystal had been rotated through 180°. 

 The adjustment was carried out finally so that the distance of the 

 face from the axis of rotation was less than .01. mm. 



An optical method was used to test if the crystal face was- 

 vertical. By illuminating the slit S^ it was possible to view, 

 in a telescope, both the slit itself and its image reflected in the 

 crystal face. When the face was vertical, the slit and its image- 

 were parallel for all positions of the crystal. 



The slit Si was then so turned, and the crystal face was so placed 

 that the light from the slit passed along the crystal face both 

 before and after the crystal was rotated through 180°. This 

 adjustment was carried out by means of a telescope, and it en^ 

 sured that the ray from the centre of S^ passed through the 

 axis of rotation. The sides of the slit Sg were then placed sym- 

 metrically with respect to those of S, . In the experiments the 

 widths of the slits were: S^, 0.08. to 0.1. mm. ; S2, 0.6. mm. 



The focal spots on the targets of both tubes were clearly- 

 marked. The focal spot in the case of the Coolidge tube was illu- 



