342 Profs. Live tug and Dewar, On some [April 30, 



measurement, we have a small photographic slide which holds a 

 plate one inch long by half an inch wide. When in use this slide 

 takes the place of that portion of the instrument which carries the 

 pointer and eye-lenses. A sliding shutter exposes only the lower 

 half of the plate which is exactly opposite the reflecting prism, 

 and when the exposure of that half of the plate is completed, the 

 plate which is held in a small drum is revolved through 180° about 

 an axis pei'pendicular to its plane, and thus what was before the 

 upper side becomes the lower. The second half of the plate is now 

 exposed and thus two images of the line are impressed on the 

 plate at equal distances on either side of the axis about which the 

 plate was turned. This axis in fact takes the place of the cross 

 wires or pointer as the point of reference; and the distance of the 

 line from it has to be deduced from the distance between the two 

 images of the line measured under a microscope by a micrometer, 

 and has then to be reduced to arc and added or subtracted from 

 the observed angle of inclination of the grating. Two sets of 

 photographs of the strong magnesium line taken in this way gave 

 the wave-length 2S5T9 and 28517 respectively; the wave-length 

 found by the old method being 28520. This line is always diffuse, 

 so there is room for some error in the determination of its middle. 

 The eye-piece was constructed for us by Mr A. Hilger, who 

 carried out our instructions very carefully. 



(2), On some modifications of Soretfs fluorescent eye-piece, By 

 Professors Liveing and Dewar. 



If the eye-piece of the spectroscope be removed and a plate 

 of uranium glass- substituted there is no difficulty in seeing, in a 

 dark room, the brightest ultra violet lines by the fluorescence, and 

 approximate measures of the deviation of them may of course 

 be made. But as soon as you bring an eye-glass to bear on the 

 image and try to take exact measures it becomes very tiresome. 

 All extraneous light has to be excluded, and unless the slit is very 

 wide the fluorescent light is in most cases so faint that it is barely 

 possible to see the pointer without losing sight of the line to be 

 observed. 



In Soret's eye-piece the eye-glass is moveable about a vertical 

 axis so that the fluorescent plate can be viewed obliquely from 

 one side and so stray light coming down the tube of the telescope 

 does not reach the eye. It occurred to us that it would be better 

 to look down on the fluorescent plate obliquely from above. The 

 spectrum and pointer would then be foreshortened in a vertical 

 plane only and observation facilitated. Further, if the fluorescent 

 screen were in form of a prism of small angle with one face per- 

 pendicular to the axis of the telescope and the other perpendicular 

 to the axis of the eye-lenses, the best conditions would be secured 



