Effect oj the Ultra-violet Spectrum. 



295 



two different portions of the spectrum, 

 ment of the detector is shown in fig. 3. 



Fisr. 3. 



The actual arrange- 

 It was circular in 



section except for the slit G, through which the light entered, 

 which was vertical. The light having entered through a 

 quartz plate CC passed through a silver wire gauze A A and 

 fell upon the zinc plate ZZ, distant 6 mm. from the gauze. 

 The plate ZZ was connected by a brass rod with the electro- 

 meter, and the silver gauze was kept at a potential of 400 

 volts by being connected through the outside brass tube to a 

 battery of small storage-cells. 



To prevent electrical leakage a tube BB kept constantly 

 at zero potential was inserted. The shaded portions repre- 

 sent ebonite which was fastened by black wax to the brass 

 tubes. All the joints were made air-tight and the apparatus 

 was exhausted to a pressure of 4 mm. through a tube (not 

 shown in the figure). 



The method of actually carrying out an experiment was as 

 follows : — Suppose the effect of light of a given refractive 

 index n was to be measured. As the focal lengths of the 

 lenses D, D (fig. 2) vary with the refractive index it was 

 necessary to find the focal length corresponding to n from 



the formula /oc — — - (the extreme values of / were 36 cm. 

 J n— 1 v 



for ?i = l*54 and 27*7 cm. for n = l*71, this being the range 

 over which electrical effects were detected). The lenses 

 were then focussed for parallel light for the particular value 

 of/. All experiments were conducted at minimum deviation 

 and the angles of incidence and deviation were calculated for 

 this value of n for minimum deviation ; which gave the 

 proper setting for the prism and receiver. With this 

 arrangement the ray of refractive index n is finally brought 

 to a focus at the centre of the slit of the detector. In order 

 to obtain a measurable effect it was found convenient to have 

 this slit about 5 or 6 mm. in width (5*5 mm. actually) and so 

 other rays slightly out of focus were also gathered in on 

 either side of the central ray. If the amount of spectrum 



