302 



THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



are absorbed, and their energy, disappearing in 

 its original form, reappears as mechanical mo- 

 tion. 



Dr. Carpenter next affirms 1 that I committed 

 myself " explicitly to the doctrine that the radi- 

 ometer is driven by light." Later on I am ac- 

 cused of showing some lingering unwillingness to 

 surrender this position ; and I am then gravely 

 censured for not knowing that heat causes the 

 movement of the radiometer. Now, what are 

 the facts ? Let my own written words speak for 

 me. 



In 1873 I published 2 the description of an 

 experiment proving that every ray of the spec- 

 trum produced repulsion : the maximum action 

 being in the extreme red. In March, 1875, I 

 wrote : 3 



" Although I most frequently speak of repul- 

 sion by heat, it must be clearly understood that 

 these results are not confined to the heating rays 

 of the spectrum, but that any ray, from the ultra- 

 red to the ultra-violet, will produce repulsion in a 

 vacuum." 



So much for my earlier ideas on this subject. 

 What are my later views regarding which, ac- 

 cording to my critic, I still show " some lingering 

 unwillingness to surrender my position? " 



In 1876 I wrote : 4 



" Is the effect due to heat or light ? I cannot an- 

 swer this question. The terms heat and light are 

 not definite enough. The physicist has no test for 

 light independent of heat. Light and color are 

 physiological accidents, due to the fact that a small 

 portion near the middle of the spectrum happens 

 to be capable of affecting the retina of the human 

 eye. There is no real distinction between heat 

 and light ; all we can take account of is difference 

 of wave-length." 



After describing experiments with a pure so- 

 lar spectrum, and giving numerical values for the 

 motion-producing powers of the various colored 

 rays, I continue : 



" A comparison of these figures with those usual- 

 ly given in text-books to represent the distribution 

 of heat in the spectrum will be a sufficient proof 

 that the mechanical action of radiation is as muoh 

 a function of the luminous rays as it is of the dark 

 heat-rays." 



Dr. Carpenter then accuses me of attributing 

 the movement of the radiometer to light. The 



' P. 245. 



2 " Philosophical Transactions," vol. clxiv.. p. 518. 



3 Ibid., vol. clxv.. p. 526. 



4 Ibid., vol. «lxvi., pp. 8C0-362. 



very contrary is the case. I have always sought 

 to guard against this misconception, insisting 

 that every ray of the spectrum, visible or invis- 

 ible, must cause motion. Hence I call the in- 

 strument the radiometer — ray-measurer. Those 

 who most persistently deny that light occasions 

 the movements, curiously enough continue to use 

 the term " %/rt-mill." 



Dr. Carpenter introduces an account of an 

 experiment I showed at the Royal Institution, on 

 the evening of the 11th of February, 1876, with 

 the words — "This he called ' weighing a beam of 

 light.' " Now, my actual words at the lecture 

 were : ' 



" I want to ascertain the amount of pressure 

 which radiation exerts on a blackened surface. I 

 ■will put a ray of light on the pan of a balance, 

 and give you its weight in grains. For I think in 

 this institution and before this audience I may be 

 allowed a scientific vse of the imagination, and 

 may speak of weighing that which is not affected by 

 gravitation.' 1 ' 1 



The italicized words render it evident that I 

 was only speakiug figuratively; and not, as Dr. 

 Carpenter wishes to make it appear, that I con- 

 ceived light to be a material substance. 



Another misstatement follows on the next 

 page, where Dr. Carpenter pronounces it "as 

 pure an assumption on Mr. Crookes's part to af- 

 firm that the mechanical action exerted by two 

 flames of different kinds would measure their 

 relative illuminating powers, as it would have 

 been to say that their heating action would be 

 proportional to their illuminating action, which 

 we know perfectly well not to be the case — the 

 gas-flame, as every one knows, having a much 

 greater heating power than the candle-flame, in 

 proportion to the light it gives." 



Once again Dr. Carpenter omits part of my 

 explanation. I will assume that he has read a 

 portion of the description of the photometric 

 experiment he criticises, given by me in the 

 " Proceedings of the Royal Society," No. 167, 

 1876. Why did he not read the next sentence, 

 beginning? — 



" By interposing screens of water or plates of 

 alum, and so practically cutting off all the dark 

 heat, the actual luminosity is measured." 



Or perhaps he gained his information from my 

 Royal Institution lecture. 2 In this case he must 

 have read the following remarks : 



1 " Proceedings of the Koyal Institution," Febmaryll, 

 1876, and Quarterly Journal of Science, April, 1876, p. 

 250. 



2 Loc. cit. 



