190 TRANSACTIONS OF THE [mAR. 1, 



it is hoped that a very complete and satisfactory series of tem- 

 peratures can be obtained, owing to the generous public spirit 

 of the Forest Oil Co., who have practically placed the well at 

 the service of science. It is hoped to sink it still deeper. 



At 9:50 the Section adjourned. 



William Hallock, 



Secretary. 



THE NEW FLICKER PHOTOMETRY. 



By F, L. Tufts. 



CONTENTS. 



Page. 



Historical Introduction 190 



Apparatus Used 197 



The Phenomenon of Flickering 198 



The Measurement of the Luminosities of Colored Disks 201 



Flickering is Found to be Independent of Color 206 



Summary 211 



Historical Introduction. 



The methods at present used to compare the luminosities of 

 two sources of light nearly all depend upon the ability of the 

 eye to detect very slight differences in the luminosities of two 

 optically adjacent surfaces. The following is, in outline, the 

 general method employed. 



Two optically adjacent surfaces are illuminated, the one by a 

 source of light, A, the other by a source B, to be compared with 

 A. The light radiated to its surface by B is then varied accord- 

 ing to some known law until the two surfaces appear to the eye 

 to be of the same luminosit3^ The ratio of the luminous in- 

 tensity of B to that of A can then be calculated according to the 

 law of variation used. 



The many forms of photometers in which the general method 

 just outlined is used differ as to the nature and position of the 

 luminous surfaces and also as to the method employed to varj' 

 the amount of light radiated to these surfaces. 



Bouger* used two white surfaces illuminated respectively 

 by the two sources of light to be compared. The surfaces were 

 at different distances from the observer, but so placed as to be 

 optically adjacent. The distance between one of the surfaces 



* Essai d' Optique. 1729. 



