On Resolving Power of Prism Trains. 355 



more than half the true value, while it made the value of e 

 twice as great as it ought to have been. 



If we know the value of e, we can at once deduce the 

 number of molecules in a c.c of gas at 0° C. and 760 mm. 

 pressure. For if N is this number, then, since e is the same 

 as the charge on the hydrogen ion in the electrolysis of 

 solutions, 



N«=l-22xl0 10 

 since £ = 3'4xl0- lu , N = 3'6xl0 19 . 



This number is well within the limits of the various deter- 

 minations made by the methods of the kinetic theory. The 

 above method for determining N has the advantage of not 

 involving a knowledge of the shape of the molecules, or any 

 assumption as to the nature of the effects produced when two 

 molecules come into collision. 



In making the experiments described in this paper, I have 

 had the help of my assistant Mr. Everett. 



XXX 1Y. On the Effect of Absorption on the Resolving Power 

 of Prism Trains^ and on Methods of Mechanically Compen- 

 sating this Effect. By F. L. 0. Wadsworth*. 

 [Plate VIII.] 



IX previous investigations t of the resolving-power of prism 

 spectroscopes, it has been generally assumed, 1st, that 

 the illumination is uniform over the wave-front passing 



* Communicated by the Author. The general investigation on resolv- 

 ing-power, of which the present paper forms a part, was begun nearly 

 eight years ago, but the work has been much interrupted and delayed by 

 other more imperative and immediate demands upon the writer's time 

 and attention. The publication of various parts of the work has there- 

 fore been more scattered and irregular than might otherwise have been 

 the case. Since this paper was written, Prof. Campbell, with whom I 

 fortunately had an opportunity of discussing it, has called my attention 

 to a paper on the same subject that was published about a year ago by 

 Dr. Reese of the Lick Observatory Staff. On examining this paper 

 (Astrophys. Journ. xiii. p. 199, April 1901) I find that Dr. Eeese has 

 deduced the general equation corresponding to (11) of this paper (for 

 5 1= 0), but has confined his investigations of the effects of absorption 

 to the particular case where the value of /3 is very small. Hence his 

 conclusion (p. 206) " that the resolving power of the Mills Spectrograph 

 is diminished by less than one half of one per cent, by the absorption of 

 the prisms " is incomplete, and for that reason erroneous and misleading 

 as a general conclusion. I have therefore allowed my own paper to 

 stand as it was originally written, acknowledging Dr. "Reese's priority 

 of publication as above. 



t See Eayleigh, Phil. Mag. vol. viii. pp. 261-264 ; vol. ix. pp. 49-56 ; 

 vol. xlii. p. 167 ; Enc. Brit. vol. xxiv. pp. 430-439 ; Schuster, ibid, vol. 

 xxii. pp. 373-374; also papers by the writer, Astrophysieal Journal, vol. 

 i. p. 52 ; vol. iii. pp. 176 and 321 ; vol. iv. p. 54 ; vol.* vi. p. 27 : vol. xvi. 

 p. 1 ; Phil. Mag. vol. xliii. p. 317. 



