22 Mr, Robert Sabine on a 



already been led by my examination of the observations of 

 other physicists on the passage of electricity through rarefied 

 gases. The maximum attained by the current-intensity at a 

 certain pressure of the air when a current traverses a space of 

 rarefied air is not due in any way, as it has been generally 

 assumed to be, to the resistance r Y l of the air having its mini- 

 mum at that pressure, and afterwards increasing in amount 

 with the increase of rarefaction, but really to this — that the 

 sum e + r-J, then possesses its minimum value. With the con- 

 tinuation of the rarefaction the resistance r x l continues to dimi- 

 nish, while e incessantly increases. Consequently the circum- 

 stance mentioned above, namely that the induced current 

 possesses its maximum value at a certain pressure of air, gives 

 no support to the allegation that in highly rarefied air or in a 

 vacuum the resistance is sufficiently great to prevent the cur- 

 rent passing. Here it is not the resistance of the gas, but the 

 electromotive force e x increasing with the rarefaction and con- 

 nected with the electrodes, that presents an obstacle to the 

 passage of the current. Every thing is in favour of the 

 hypothesis that vacuum opposes a very feeble resistance to 

 the propagation of electricity. One can therefore, without the 

 employment of electrodes, by induction at a distance, or by 

 friction at the surface of a tube in which the air is sufficiently 

 rarefied to render the passage of a strong induction-current 

 between the electrodes impossible, easily excite in that tube 

 an electric motion sufficiently considerable to produce a sen- 

 sible development of light. Now this would be impossible if 

 highly rarefied gas or a vacuum were an insulator. 



II. On a Wedge-and-Diaphragm Photometer. 

 By Robeet Sabine*. 



THE photometer described by me in a paper contributed to 

 the last Meeting of the British Association at South- 

 ampton! was based upon the weakening of the light to a 

 constant value by the interposition of sheets of some absorbing 

 material as far as could be, and finally obtaining a balance by 

 varying 'the distance of the photometer from the light to be 

 measured. 



I find it, however, frequently in practice more convenient 

 to keep the photometer at a constant distance from the source 

 of light, and to effect a balance by the gradual increase of the 

 thickness of the absorbing material only, using at the same 



* Communicated by the Author. 

 t Electrical Review, vol. xi. p. 197. 



