PROCEEDINGS OF THE ACADEMY AND AFFILIATED 



SOCIETIES 



PHILOSOPHICAL SOCIETY OF WASHINGTON 



830TH MEETING 



The 830th meeting was held at the Cosmos Club, February 14, 1920. 

 President Sosman presided and 38 persons were present. 



The first paper by Messrs. W. F. Meggers and Paul D. Foote 

 on A new microphotometer for photographic densities was presented by 

 Mr. Meggers. 



The new microphotometer for measuring photographic densities is 

 essentially the micropyrometer described in the Bulletin of the Bureau 

 of Standards (9: 475. 1913) except that a microscope of higher power 

 is used. The photographic plate is mounted just below the objective 

 of the microscope on a horizontal bed movable with a graduated screw 

 and is illuminated beneath by an intense beam of light from a tungsten 

 ribbon lamp. Light transmitted by a small portion of the photographic 

 plate, the image of which is adjacent to that of the tip of the pyrometer 

 lamp, is matched with equal filament brightness by adjusting the cur- 

 rent through the lamp. The ammeter readings are readily translated 

 into measurements of photographic densities. If the tip of the pyrom- 

 eter lamp filament be regarded as a filar in the eye-piece, the relative 

 positions or wave lengths of spectral lines on a plate are measured at 

 the same time that their photographic densities are measured. 



An example of the use of this microphotometer for measuring wave 

 lengths and densities of spectral lines is given and it appears that the 

 visually estimated intensities of such lines are a geometric series in 

 accordance with Fechner's law. 



Illustrations of this microphotometer's application to the measure- 

 ment of energy distribution in a broad spectral line and to the meas- 

 urement of spectral sensitivity of a photographic plate are given. 



Measurements of photographic density when the plate is illumin- 

 ated by parallel light are found to be much larger than when the illum- 

 ination is diffuse. This experience calls attention to the importance 

 of specifying the character of the illumination when measurements are 

 made on light transmission of diffusing media. The paper was illus- 

 trated by lantern slides. 



Discussion. — The paper was discussed by Messrs. Sosman and Hum- 

 phreys. 



The second paper, also by Messrs. FooTE and Meggers, was pre- 

 sented by Mr. FooTE. The title was Atomic theory and low-voltage 

 arcs in caesium vapor. The paper was illustrated by lantern slides. 

 Full publication may be found in Phil. Mag. Series 6, 40: 80. 1920. 



The above paper was discussed by Messrs. Sosman and Tuckerman. 



497 



