On the Aherration of the Concave Gratimj. 119 



by J. J. Thomson "^ and by nie f. I have shown that the 

 normal cathode drop of potential in the olovvino- discharge 

 (glow current) is equal to the ionizing potential of the positive 

 ion relative to the cathode metal. Furtber, J. A. McClelland % 

 has already made an observation which is similar to the 

 l)henomenon studied by Townsend, hut more simple. There 

 are produced by heating positive and negative ions at the 

 snrlace of a wire which is placed in the axis of a metal 

 cylinder ; the positive or negative ions are driven by an 

 electric field through the rarefied gas to the inner side of the 

 cylinder ; if the freely passed potential-difference is large 

 enough, they are able to ionize by collision the gas at the 

 inner surface of the cylinder. I have shown [Ann. d. Phys. 

 viii. p. 826, 1902) that in a set of measurements of McClelland 

 it is the positive ion which produces new ions by collision. 



I remain, Gentlemen, 



Yours very truly, 

 Gottingen, 17 April, 1903. J. Stark. 



XV J. On the Aherration of the Concave Grating^ lohen nsed 

 as an Objective Spectroscope. By F. L. 0. Wadsworth§. 



THE question of the aberration of concave gratings was 

 first investigated by Rowland || and Glazebrook^. The 

 results obtained by them refer only to the case in which the 

 ruled surface is spherical in form, and is so mounted that 

 both it and the source of light lie on the circumference of a 

 circle whose diameter is /5, the radius of curvature of the 

 spherical surface. When so ruled and mounted the aberra- 

 tion is symmetrical in form, and is so small that for gratings 

 of the angular aperture usually employed we may use re- 

 solving-powers as high as 4,000,000 before the definition 

 begins to be affected. Since the resolving-powers ordinarily 

 employed in the largest concave-grating spectroscopes are 

 only from one to five per cent, of this limiting value, the 

 effect of aberration can be safely neglected, even when the 

 angular aperture of the grating is increased from two to 

 three times. 



* J. J. Thomson, Phil. Mag. (o) 1. p. 282 (1900). 

 t J. Stark, Ann. d. PJujs. pp. 431, 43o, 923 (1902), viii. p. 826 (1902). 

 X J. A. McClelland, Proc. Cambr. Phil. Soc. xi. p. 296 (1901). 

 § Communicated by the Author. 

 II Phil. Mag. [o] vol xvi. p. 197. 



51 Ibid. vol. XV. p. 414; vol. xvi. p. 337. See also article on the 

 " Wave Theory," by Lord Rayleigh, Enc. Brit. vol. xxiv. p. 439. 



