684 



of tlie cylinder, enable ns to make the liglit strike the mercury at 

 the required angle of incidence. The mercury is conveyed into the 

 dish by a tube in the upper surface of the cylinder. This bronze 

 cylinder is attached lo the middle of the goniometer circle. A vertical 

 sledge makes it possible to place the mercury surface so, that the 

 axis of the goniometer lies in it. By means of an horizontal sledge 

 the dish may be placed so, that (he axes of the incident and the 

 reflected beam of light pass through the middle of the glass windows. 

 The cylinder can be exhausted and filled with air, that has been 

 dried with calcium-chloride, sulphuric acid, potassium-hydroxide and 

 phosphor-pentoxide. 



When the position of the movable prism of the compensator, 

 immediately after the formation of the mercury surface is called 

 c„, that t seconds later c, and the final position c^, a very plausible 

 supposition on the increase of the thickness of the layer of air and 

 its optical influence leads to the ditferential equation : 



dt V =0 / 



So that 



In this k will l)e proportional to the pressure of the air. This 

 supposition is confirmed by series of observations, made at a pressure 

 of one and of half an atmosphere, which are graphically represented 

 in fig. J. There have been traced six curves for different values of ^^ 

 It appears clearly from the traced lines within what limits the value 

 of k lies for the two series of observations. As value of c^ — c„ has 

 been taken 0.25, viz. the change of the readings in dry air after 

 24 hours. When (he cylinder is exhausted, no change in the compen- 

 sator-readings can be demonstrated even after 8 hours. Compare 

 the line . . ., which indicates the observations in the exhausted cylinder. 

 Here follow the means of three series of observations in dry air 

 for a pressure of an atmosphere. 



Time Angle of Incidence 79°46' 



Co = 56.84 C-C\ 



1 hour C = 56.863 0,023 



1 „ 56.89 0.05 

 14 hours 56.92 0.08 



2 „ 56.918 0.078 

 2^ „ 56.94 0.10 



3 „ 56.965 0.125 

 3è „ 56.975 0.135 



4 „ 56.972 0.132 



