174 



SCIENCE. 



[N. S. Vol. XVI. No. 396. 



tion, phase angles and power, may be 

 undertaken by the student with ease. 



Measurement of the Intensity and Pressure 

 of Badiant Energy: E. F. Nichols and 

 G. F. Hull, Dartmouth College. Pre- 

 sented by Professor PIull. 

 This paper was supplementaiy to a paper 

 on the same subject which was read at the 

 Denver meeting of the American Associa- 

 tion for the Advancement of Science in 

 August, 1901. In the earlier paper it was 

 shown that the 'gas action' due to a beam 

 of light falling on a torsion radiometer had 

 been approximately eliminated and that the 

 'light pressure' had been measured to the 

 same degree of approximation; but that in 

 the comparison of the experimental value 

 of the light pressure with that deduced 

 from the Maxwell-Bartoli formula, using 

 the value of the energy as measured, a dis- 

 crepancy of about twenty per cent, was 

 found. 



An analysis of the earlier work showed 

 an error in the energy measurement due to 

 an error in the resistance of the bolometer 

 used. A new value of this resistance was 

 found by the potentiometer method and by 

 the application of theory to the determina- 

 tion of •- the resistance of a circular disc. 

 The new value of the energy gave an agree- 

 ment between the Maxwell-Bartoli formula 

 and experiment, to about five per cent. 



Later a more satisfactory method of 

 measuring the energy was used, in which 

 was observed the change of temperature of 

 two thermojunctions of fine iron and con- 

 stantin wire, placed inside a small silver 

 disc upon which the radiation fell. The 

 thermoelement was calibrated by immers- 

 ing the silver disc in baths of kerosene at 

 different temperatures. The average error 

 in the measurement of the energy was con- 

 sidered to be less than one per cent. 



By analysis of the earlier data on light 

 pressure it was shown that the gas action 

 had been eliminated approximately from 



the ballistic values, but was present in the 

 statical readings. Later and more accurate 

 experiments confirmed this result. The 

 new value of the radiation pressure was 

 found to agree with that derived from the 

 Maxwell-Bartoli formula, using the new 

 energy determination, to within a few per 

 cent. For greater accuracy it will be neces- 

 sary to measure the absorption and reflec- 

 tion coefficients of the surfaces used. 



On the Penetration of Light into the Barer 

 Medium in the Case of Total Beflection: 

 E. E. Hall, Cornell University. Read 

 by title. 

 This paper will appear later in the 



Physical Beview. 



An Improved Form of Torsion Badiometer: 

 E. F. Nichols, Dartmouth College. 

 A new radiometer case, somewhat larger 

 than that of similar instruments earlier 

 described, was shown. The advantage of 

 the new form lies wholly in the support 

 carrying the suspension, which is so de- 

 signed that the length of the quartz fiber 

 may be easily changed and the sensitive- 

 ness and period of the instrument altered 

 through a wide range. All adjustments 

 may be made without removing the sus- 

 pension from the case, so that the danger 

 of breaking the fiber is reduced to a mini- 

 mum. 



The Optical Properties of Iodine: "VV. W. 



CoBLENTZ, Cornell University. Read by 



title. 



The paper will appear in the Physical 

 Beview. 



The Emission of a Bighi Vibrator and the 

 Measurement of the Length of Electnc 

 Waves hy the Interferometer: H. R. Wil- 

 LAED and L. E. Woodman, Dai-tmouth 

 College. • Presented by E. F. Nichols. 

 The paper dealt with the radiations emit- 

 ted from a Righi vibrator which were 

 studied in the first instance by resonance 



