228 Prof. J. J. Thomson on the Electrical 



easily visible. Lord Rayleigh (' Collected Papers,' vol. iv. 

 p. 128) estimates that from a standard candle the energy 

 in the luminous radiation is about 5 X 10 5 ergs per second. 

 Hence the quantity of energy crossing one square centimetre 

 at a distance of 100 metres from the candle would be 

 5 x 10 5 /47r x 10 8 or 4 x 10 -4 erg per second, considerably 

 less than the energy in the luminous radiations sent out by 

 the cathode rays in the case just considered ; so that this 

 light ought to be bright enough to affect the eye. The 

 intensity o£ the light could be materially increased by ac- 

 celerating the corpuscles given out from the lime by an 

 electric field. If the velocity were increased tenfold, the 

 intensity of the light would be increased one hundredfold. 



The energy in the light in the octave of wave-lengths from 

 3*5 x 10"° cm. to 1*75 x 10~ 5 cm. would be twice that in the 

 octave corresponding to luminous radiation, and until we 

 approached the wave-length 6*3 x 10 -6 cm. (supposing the 

 velocity of the corpuscles to be 10 8 cm./sec), the energy in 

 each succeeding octave would double as the wave-length 

 diminished. Thus we must have here a source of ultra-violet 

 light cf much smaller wave-length than any yet investigated, 

 and by increasing the velocity of the corpuscles to 10 9 cm./sec. 

 or further, we ought to be able to produce light with a wave- 

 length of less than 10 ~ 8 cm. ; in fact, to completely fill up 

 the gap between the infra-red radiation and liontgen rays. 

 I am at present engaged in experiments with this object, 

 making the corpuscles given out by hot lime impinge against 

 a solid obstacle. 



We see from equation (8), t 2 being large compared with a, 

 that the total amount of energy radiated per unit time is equal to 



2 Ne 2 u 2 



3 wVa ' 

 or, since a is the time of a collision, to 

 2NA 2 f 1 



3 



1 1 _!• 



I space travelled by light in the time of a collision J 



Thus the energy in the visible part of the spectrum will be 

 to the whole of the energy radiated as the thickness of the 

 pulse in the Rontgen radiation is to a length comparable 

 with the wave-length of visible light. When the cathode 

 particles are moving at a speed of 10 9 cm. per sec, the 

 thickness of the Rontgen pulse is only about one-thousandth 

 part of the wave-length of sodium light. Hence only about 



