102 Professor Ernest Fox Nichols [May 12, 



WEEKLY EVENING MEETING, 

 Friday, May 12, 1905. 



The Earl of Rosse, K.P. D.C.L. LL.D. D.Sc. F.R.S., Vice- 

 President, in the Chair. 



Professor Ernest Fox Nichols. 



Columbia University, New York, 



The Pressure due to Radiation. 



[abstract.] 



The first speculations upon a possible pressure due to radiation were 

 suggested by the behaviour of comet tails. 



Early in the sixteenth century, Pierre Apian announced that the 

 tails of comets were always directed away from the sun, and a century 

 later Kepler maintained that this repulsion was due to the pressure 

 of sunlight. On the corpuscular theory of light it seemed plausible 

 that the finely divided and very attenuated matter supposed to con- 

 stitute comet tails, might experience a repulsion of which ordinary 

 bodies gave no evidence. 



The three intervening centuries from Kepler's time to our own, 

 exhibit a long and very interesting record of conflicting opinions, and 

 the account of many curious and inconclusive experiments. 



In 1900-1901 the first experiments giving undoubted evidence of 

 the existence of a pressure due to radiation were announced in- 

 dependently from Moscow by Professor Lebedew, and from New- 

 Hampshire by Nichols and Hull. 



Maxwell had earlier maintained that radiation pressure was a 

 necessary consequence of the Faraday-Maxwell electromagnetic theory 

 of light, and after him, Bartoli was convinced that the same result 

 should follow from the laws of thermodynamics. In computing the 

 ratio of the pressure to the intensity of the radiation producing it. 

 Maxwell and Bartoli were in exact agreement. 



In the experiments of Nichols and Hull in which both the 

 pressure and the intensity of a beam of light were measured, the 

 ratio was found to agree with the Maxwell-Bartoli theory to within 

 one part in a hundred. As the limit of accuracy of the observations 

 was of this same order, the experimental verifications of the Maxwell- 

 Bartoli theory may be accepted as complete. 



Nichols's and Hull's experiments Avere described by the lecturer, 



