188 Dr. ^X . Watson on the Construct ion and 



the calculated values of X are represented, the scale being 

 the same as in fig. '2. 



The constant B varies nearly as the pressure, while C 

 increases rapidly with diminishino^ pressure. If X^ is the 

 maximum value of the intensity, then X^ = B, so that X^ 

 varies as V/'- 



C is equal to —r^ ^^^ ^~ T' consequently, since C 



increases as the pressure diminishes, it follows that /3 the co- 

 eflScient of recombination must rise with increasing pressure. 

 This result is in agreement with Langevin's measurements^ 

 of the coefficient of recombination of the ions produced in 

 air by Rontgen rays at low pressures. 



The experiments described in this paper were done at the 

 Cavendish Laboratory, and I wish to say that my best thanks 

 are due to Prof. J. J. Thomson for his kindly interest and 

 advice given durino- the course of the work. 



XXII. JS'ote on the Construction and Attachment of Thin 

 Galvanometer Mirrors. By W. Watsox, D.Sc, F.B.S., 

 Assistant Professor of Physics, Pot/al College of Science^ 

 London'\. 



AS has been pointed out bv several persons (Ravleigh, 

 Phil. Mag. XX. p. 360/1885; Threlfall, Proc.^hys. 

 Soc. x^a. p. 205, 1898) there are distinct advantages in in- 

 creasing the sensitiveness of the optical arrangements used 

 to measure the rotation of a galvanometer needle rather than 

 increasing the electrical sensitiveness, i. e. the -angular de- 

 flexion produced by a given small current. Excessive elec- 

 trical sensitiveness implies a very weak controlling force. 

 Such a weak controlling force involves a long period, and 

 also disturbances produced by convection currents in the air 

 and tremors in the supports of the instrument. 



In order to increase the optical sensitiveness of a galva- 

 nometer it is necessary to increase the diameter of the mirror, 

 while, to allow of its being carried by a fine suspension and to 

 keep the period low, the weight of the mirror must be made as 

 small as possible, vet at the same time its figure must remain 

 good. With the ordinary form of mirror, consisting of a 

 disk of glass silvered on the back, it is almost impossible to 

 obtain a really thin mirror with a good figure unless the area 

 of the mirror is excessively small. In the first place, the 

 grinding and polishing of a very thin slip of glass is an 



* Theses presentees a la Faculte des Sciences, Paris, pp. 150-152. 

 t Commimicated by the Physical Society ; read May 8, 1903. 



