On Screens Transparent only. to Ultra- Violet Light. 257 



coefficient of absorption equal to that of rock-salt and a thermal 

 conductivity equal to that of cork. Curves I. and II. give the 

 distribution of temperature when the average temperature is about 

 30° C, Curve I. representing the steady state when the boundaries 

 are totally reflecting, and Curve II. when they are black. Curve III. 

 represents the steady state for totally reflecting boundaries when 

 the average temperature of the plate is about 360° C. This curve 

 shows the approach to the steady state, when the transmission of 

 heat is entirely effected by radiation, thermal conductivity being 

 neglected. The temperature distribution in the limiting case being 

 uniform, but discontinuous at the surfaces. The numbers from 

 wdiich the curves are plotted are given in the. following Table, which 

 includes also under column IV. the case of black surfaces, with an 

 average temperature of 360°. It is only necessary to give the 

 temperatures near the colder bounding surface, as the distribution 

 is symmetrical at both sides and uniform near the centre of the 

 plate. The columns headed Temperature Differences represent 

 the difference between the temperature at any point and the tem- 

 perature of the colder surface ; the complete temperature change 

 in the plate is supposed to be 1°. 



Distance from 



Colder 



Boundary 



in cms. 





Temperature Differences. 





I. 



11. 



III. 



IV 



•01 



•075 



•042 



•240 



•127 



•02 



•129 



■074 



•330 



•178 



•04 



•196 



•117 



•384 



■212 



•05 



•217 



•133 



•388 



•221 



•10 



•274 



•186 



•400 



•253 



•20 



•335 



•267 



•425 



•315 



XXIII. On Screens Transparent only to Ultra-Violet Light 

 and their Use in Sp>ectrum Photography . By "R. W. Wood, 

 Professor of Experimental Physics, Johns Hopkins Uni- 

 versity*. 



[Plates III. & IV.] 



ANYONE who has repeated TyndaH's beautiful lecture 

 experiment of kindling a pine stick in the dark heat 

 focus of a burning-glass, concentrating light from which the 

 visible radiations have been removed by means of a solution 



* Communicated by the Author. 

 Phil. Mag. S. 6. Vol. 5. No. 26. Feb. 1903, S 



