740 Dr. C. Statescu on Dispersion of Carhon Dioxide 



(991*3 mm.) the change in deviation (AD^is expressed by 

 the formula 



AD=A<2/2M"B, 



because the rays passed twice through the prism. Between 

 O'S fi and 13 fi rock-salt is quite transparent, and it was in 

 this region that I measured the dispersion o£ carbon dioxide. 



Two galvanometers were used, one, less sensitive, for the 

 region up to 5 /x, and, for the remainder, a more sensitive one. 



In order to make the linear displacement, which is pro- 

 portional to the pressure, reasonably large, I worked with 

 changes of pressure of the order of two atmospheres. Between 

 the limits I took, the rock-salt plates underwent no deforma- 

 tion whatever. 



Some details of the determination of the displacement Ad 

 must be given. Ad for each wave-length is the distance 

 between the positions of the thermopile for the maximum 

 deflexion of the galvanometer for two different pressures. 

 These maximum deflexions were found from the graphs 

 plotted between galvanometer deflexions and the positions 

 of the thermopile. For each curve six to eight positions of 

 the thermopile were taken. Each curve was plotted twice. 

 The first time the thermopile was moved in one direction 

 and the second time in the reverse way. The maximum for 

 each curve was found as follows. Tangents were drawn to 

 the curve at corresponding pairs of points. These intersected 

 above the curve. The maximum was taken to be at the 

 point at which the best straight line drawn through the 

 intersecting points cut the curve. I found a certain lack of 

 symmetry in all the curves. I was able to correct for this 

 to some extent by attaching greater weight to the accuracy 

 of the tangents drawn from points near the foot of the curve 

 than from points near the top. 



After readings had been taken for a series of wave-lengths 

 at a certain pressure, verified at the end of the observations, 

 and at a temperature read from time to time, the same 

 observations were repeated at a different gas pressure. Each 

 series of observations was preceded by one made by the eye 

 in sodium light, so that the refractive indices might be 

 given in their absolute values. 



1 was able to gauge the accuracy of my observations from 

 the analyses made by Kayser and Runge* of the errors in 

 their dispersion experiments. They found that a difference 

 of temperature of one-tenth of a degree or a difference in 

 pressure of one-tenth of a millimetre of mercury had very 

 small bearing on the fourth significant figure of the resuJt. 

 * Kayser and Runge, u Die Dispersion der Luft," Abh. der K. Preuss. 



