THE PRESSURE DUE TO RADIATION. 

 By E. F. Nichols and G. F. Hull. 



Presented December 10, 1902. Received February 5, 1903. 



Contents. 



PAGE 



Historical Literature 559 



Outline of Preliminary Work 50*2 



The Work of Lebedew 567 



Later Pressure Measurements 5G8 



The Torsion Balance 568 



Arrangement of Apparatus 570 



Methods of Observation 572 



Torsion Coefficient of Balance 573 



Reduction of Observations to Standard Lamp 573 



Static Method of Pressure Measurements 573 



Methods for the Elimination of Gas Action 574 



Ballistic Method of Pressure Measurements 577 



Pressure of Beam of Standard Intensity for Different Wave-groups .... 579 



Apparatus and Method of Energy Measurements 584 



Calibration of Silver Disc 585 



Energy of Beam of Standard Intensity for Different Wave-groups .... 588 



Peflection Coefficients of Surfaces 591 



Corrections and Final Computations 592 



Estimate of Uneliminated Gas Action in Final Values 597 



Accuracy of the Various Measurements entering into the Final Values . . . 598 



As early as 1619 Kepler* announced his belief that the solar repul- 

 sion of the finely divided matter of comets' tails was due to the outward 

 pressure of light. On the corpuscular theory of light Newton f con- 

 sidered Kepler's idea as plausible enough, but he was of the opinion 

 that the phenomenon was analogous to the rising of smoke in our own 

 atmosphere. In the first half of the eighteenth century DcMuiran and 

 DuFay t contrived elaborate experiments to test this pressure of light 



* DeMairan, Traito physique et historique de I'Aurore Borealc (Seconde Edi- 

 tion), pp. 357-358. Paris, 1754. 



t Isaaci Newtoni Opera quae Existant Omnia. Samuel Ilorsley, LL.D., 

 R. S. S. Tom. Ill,, pag. 156, Londinium, 1782. 



\ DeMarian, i.e., p. .371. This treatise contains also the nccoimts of still earlier 

 experiments l)y Ilartsoeker, p. 3f)H, and IIoml>erg, p. 369. Tiie later experiments 

 are of more historic than intrinsic interest. 



