ENERGY AND VISION. 



By p. LECOMTE du NOtJY. 



{From the Laboratories oj The Rockefeller Institute for Medical Research.) 



(Received for publication, April 19, 1921.) 



Although a large number of papers and books have been pubhshed 

 on the problems of vision (1), a very limited amount of work is to 

 be found on the minimum energy necessary to produce visual sensa- 

 tion. The classical work of Langley (2) for different wave lengths 

 the papers of Grijns and Noyons (3), Zwaardemaker (4), and Kries 

 (5), for white light are always quoted, but the figures given by differ- 

 ent authors do not always agree, discrepancies of 100 per cent, some- 

 times of 1000 per cent, being frequent, with no explanation. For 

 this reason, it was thought advisable to check all these figures, in 

 order to ascertain whence came the discrepancies. Furthermore, 

 as Langley's figures are given by himself with a certain degree of 

 approximation, and were calculated for the fight emitted by the 

 sun, we thought it would be interesting to check them by another 

 method, for another source of light, the Nernst lamp, for instance. 

 These are the reasons for carrying on this series of measurements. 



In order to give an idea of how difficult it is to iind a figure corresponding to 

 the minimum visibile for a certain wave length, we will give an example. Lang- 

 ley's figures are quoted as follows for the wave length 0.55/x, for which the human 

 eye shows a maximum of sensitivity: 



By Broca (6) 5.6 X 10-^ ergs 



By Henri and des Bancels (7) 3 . X 10~^ ergs 



whereas Langley's real figure, as given in his paper, is 2.8 X 10~^ ergs. Further- 

 more, Henri and des Bancels state on another page that 10"-^" is the order of mag- 

 nitude of the minimum energy necessary to produce the sensation of vision in 

 the green (0.55ju), and Langley (2)^ states that it is 1.0 X 10~^ (for practically 

 the same radiation, O.SS^u). In order to clear this matter up, we have to go over 

 Langley's paper carefully. Langley states and gives a solution for two different 

 problems: first, determination of the intensity of light necessary to read a table 



'^ Langley, (2), p. 23. 



743 



