20 ON THE APPLICATION OF INTERFERENCE METHODS 



In Fig. 196 the ciu-ved line gives the relation between 5 and — , and shows 



clearly that when p is less than 5 '"'" the effect of collisions has almost en- 

 tirely ceased. If we take as variables 5 and p, the results agree very closely 

 with the straight line ri-^Jo=h p. in which 5o= 0.047 (the "half- width'' of 

 the line at zero pressure in the units adopted), A* = 0.00093, and p is the 

 pressiu-e in millimeters.* 



The same results were found for the 1)lue hydrogen line, though, as 

 might be expected, these were not so consistent. 



It thus appears that in the case of hydi-ogen — and proba})ly in all other 

 cases — the ^vidth of the spectral lines diminishes toward a limit as the 

 pressure diminishes, which depends \ipon the suV)stance and its tempera- 

 tm-e ; and that the excess of width over this Umit is simply proportional to 

 the pressure. 



In general it may be said that under considerable ranges of temperature 

 and pressm-e the character of the visibihty curve remains the same; biit it 

 may be important to note that there are a number of exceptions to tliis nile, 

 among wliich the gi"een mercmy hne and the yellow sodimn line may be 

 especially mentioned. 



Thus, Fig. 20fl, Plate V., represents the visibility cm-ve iisually observed 

 for the green mercury hne, and Fig. 20c" represents that obtained when the 

 vacuum is so high that the discharge passes with difficulty, while Fig. 206 

 represents the intermediate stage. This last observation was obtained by 

 placing the mercmy in an atmosphere of hydrogen whose pressui*e could be 

 measured by a McLeod gauge. 



It might be objected that the presence of a foreign substance might of 

 itself iiffect tlie distiibution of light in the source, and therefore the form 

 of the cm-ve. In order to test this point, a series of observations of the red 

 hj'drogen was taken while the tube contained liquid mercury which was 

 heated until the mercury spectnim was at least ten times as bright as that 

 of the hydrogen. The character of the visibihty cm-ve was not perceptil)ly 

 altered. 



In tlie same series of expeiiments it was found that, provided the pres- 

 sure of the hydrogen remained constant, the effect of a change in tempera- 

 tun^ from 75^ to 140^ had no appreciable effect on the result. In this 

 connection it may be mentioned that the character of tli(» curve for the 

 gi'een mercuiy line was not essentially altered when, in place of metalhe 



• In the figure the numbers representing values of the abscissee for this line should be multipliefl by 100. 



