PROCEEDINGS OF THE ACADEMY AND AFFIUATED 



SOCIETIES 



PHILOSOPHICAL SOCIETY OF WASHINGTON 



827TH MEETING 



The 827th meeting was held at the Cosmos Club, January 3, 1920, 

 with President Sosman in the chair and 55 persons present. 



Mr. Enoch Karrer presented two papers, the first being on Diffusion 

 of light along a searchlight beam. 



The importance of the light diffused along a searchlight beam is 

 pointed out. It aids in the directing of the beam, and makes it possible 

 to use searchlight beams as land marks for service on land and in 

 the air and perhaps as beacons in the lighthouse service. On the other 

 hand, this diffusion means loss of light and interferes with the dis- 

 cerning of the target from positions near the searchlight. Data on 

 the brightness of the diffused light for various angles along the beam 

 were given as well as data on the per cent of plane polarized light in 

 the diffused light. An explanation was offered of the apparent ending 

 of the searchlight beam, and of the curving of it under certain condi- 

 tions. 



Discussion: The paper was discussed by Messrs. Crittenden, 

 Humphreys and Sosman. 



Mr. Karrer' s second paper was on The contrast sensibility of the 

 eye under low illumination. 



Data on contrast sensibility were given that were obtained under 

 conditions simulating those that actually obtain in the use of search- 

 lamps. An illuminated strip was projected on a field of known bright- 

 ness. The length of the strip was increased from zero to a length just 

 visible to the observer. The results obtained are expressed by curves 

 showing the relation between (i) strip length (visual angle) and field 

 brightness, for constant contrast between strip and field; (2) strip 

 length and contrast between field and strip, for constant values of field 

 brightness. These curves lie in groups consistent with each other, and 

 those obtained for two observers are similar. Examples were given 

 of how these data together with the data on diffused light may be 

 applied. 



Both of the above papers were illustrated by lantern slides. 



The next paper was by Mr. F. E. Wright, on The contrast sensibility 

 of the eye as a factor in the resolving power of the microscope. 



In this communication attention was directed to three factors which 

 are of importance in high-power microscope work, namely: (a) The 

 use of a polarizing prism to eliminate that part of the field light which 



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