99 



Mr. Talbot Paris and Prof. Porter : An Experiment 



(X 5461) was 534*3 degrees. To obtain a sufficiently clear 

 solution it was necessary to mix some freshly precipitated 

 and washed aluminium hydroxide with it and filter the liquid 

 warm. This removes practically all suspended matter. 



Fiff. 1. 



ma b 



h~f {Z3-ZS7- 



$' 



E 



The lens L 2 forms a reduced image of the half -wave plate,. 

 Mj on the slit Sj. The remainder of the apparatus, comprising 

 the analysing nicol N 2 , the constant-deviation prism P, and 

 the telescope T, was adapted from a spectrophotometer. The 

 graduated circle attached to N 2 was fitted with a vernier 

 attachment reading to minutes. Light of the required 

 wave-length is sorted out by rotating the prism P and 

 adjusting the slit s 2 . The telescope is focussed on the slit Sj 

 so that a sharp image of the half-wave plate is seen. 



All measurements were made with the green mercury line 

 (X5461). Further purification of the light after its passage 

 through P was found necessary, and so the direct-vision 

 spectroscope D was inserted between the eyepiece E and 

 the rest of the telescope. With the light from the arc there 

 was some diffuse light from the yellow mercury lines present,, 

 and with the spark, in addition to the diffuse yellow light, 

 some green light on each side of the mercury line, but less 

 intense. This green light was not diffuse but formed a 

 definite image of the half-wave plate. 



The spark was produced by the discharge of a large leyden- 

 jar connected with an induction-coil, and passed between a 

 platinum wire and a layer of mercury at the bottom of 

 a glass vessel, contact with the mercury being made with 

 a piece of iron wire soldered on to the end of the lead from 

 the leyden-jar. Considerable inconvenience in taking the 

 readings was caused by the irregular movements of the 

 spark, a small change in its position causing the image 

 formed by the lens L 2 to move off the slit Sj. To keep the 

 spark as much as possible in one position the platinum wire 

 was placed between two pieces of plate-glass standing about 



