182 



BEPOET — 1892. 



In order to sho-w 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 

 sei'ies of observations was made on the red hj-drogen line at varying 

 pressures, with the results shown in fig. 19a., Plate IV.' 



Prom these curves the following table was calculated : — 



Pressure in mm. 

 90 

 71 

 47 

 23 

 13 



9 



3 



5 



S 



•128 

 •116 

 •095 

 ■071 

 •056 

 •053 

 -050 

 -048 



In fig. 196 the curved line gives the relation between 8 and -, and 



-^. 

 shows clearly that when p is less than 5 mm. the effect of collisions 



has almost entirely ceased. If we take as variables 8 and j5, the results 



agree very closely with the straight line S — 8o=^12^> ^^ which 8o="047 



(the ' half-width ' of the line at zero pressure in the units adopted), 



/i;=-00093, andp is the pressure in millimetres.^ 



The same results were found for the blae 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 tem- 

 perature and pressure the character of the visibility curve remains the 



' The numbers against the curves denote pressure in millimetres. 

 2 In the figure the numbers representing values of the abscissse for this line 

 should be multiplied by 100. 



