Interference Methods to Spectroscopic Measurements. 295 



In order to show conclusively that the effect of density may 

 be neglected in the foregoing observations, as well as to 

 ascertain the law governing the broadening of spectral lines 

 by pressure or density, a series of observations was made on 

 the red hydrogen-line at varying pressures, with the results 

 shown in fig. 19 a, Plate VII.'* 



From these curves the following Table was calculated : — 



Pressure in millim. 8. 



90 -128 



71 -116 



47 -095 



23 -071 



13 -056 



9 -053 



3 -050 



5 -048 



In fig. 19Z> the curved line gives the relation between 8 



and — , and shows clearly that when p is less than 5 millim. 



the effect of collisions has almost entirely ceased. If we take 

 as variables 8 and p, the results agree very closely with the 

 straight line 8 — 8 = kp, in which 8 = '047 (the " half-width " 

 of the line at zero pressure in the units adopted), & = '00093, 

 andp is the pressure in millimetres f. 



The same results were found for the blue hydrogen-line, 

 though, as might be expected, these were not so consistent. 



It thus appears that in the case of hydrogen — and probably 

 in all other cases — the width of the spectral line diminishes 

 towards a limit as the pressure diminishes, which depends 

 upon the substance and its temperature ; and that the excess 

 of width over this limit is simply proportional to the pressure. 



In general, it may be said that, under considerable ranges 

 of temperature and pressure, the character of the visibility- 

 curve remains the same ; but it may be important to note that 

 there are a number of exceptions to this rule, among which 

 the green mercury-line and the yellow 7 sodium-line may be 

 especially mentioned. 



Thus, fig. 20 a, Plate VII., represents the visibility-curve 

 usually observed for the green mercury-line, and fig. 20 c 

 represents that obtained when the vacuum is so high that the 

 discharge passes with difficulty, while fig. 20 b represents the 



* The numbers against the curves denote pressure in millimetres, 

 t In the figure, the numbers representing values of the abscissae for 

 this line should be multiplied by 100. 



