f44 



ENERGY AND VISION 



of logarithms or to discern any arbitrary character and second, determination of 

 the minimum visibile; namely, the minimum of energy which can produce the 

 sensation of light on the retina. The results of the first determinations are 

 expressed in tables (2)- in function of the wave length and of the sensitiveness of 

 the eye, in arbitrary figures related to the apparatus and inversely proportional 

 to the energy. The results of the second determinations are expressed, on the 

 contrary (2)^ in the following way: reciprocals of calories = reciprocals of ergs 

 (let us call this Table A) , and these values stated in terms of horse-power (Table 

 B). Now the meaning of these tables is quite ambiguous, and it is not surprising 

 that authors have been mistaken in quoting them because, as they are given, 

 they are only consistent provided the first table (A) is given in ergs per § second. 

 But as the figures in Table B, being expressed in horse-power, cannot be given in 5 

 seconds (as the horse-power unit carries in itself its time unit and can only be 

 used in connection with it, namely, 1 second), the figures of Table A must first be 

 transformed into ergs per second, that is, multiplied by 2, to be identical with 

 those of Table B. This is what has probably escaped the attention of Broca, 

 and of Henri and des Bancels, and unfortunately. Broca took his figures from Table 

 B, and Henri and des Bancels took theirs from Table A, so that all the figures of 

 Broca are exactly double those of Henri and des Bancels. It is possible that 

 these authors have not been mistaken, and that one of them (Broca) reduced 

 the figures in Table A to ergs per second, whereas Henri and his coworker simply 

 took them as they were. But it is most regrettable that none of them gave any 

 indication as to the unit of time. Moreover, an important error is to be found 

 in the figures of Henri and des Bancels due perhaps to misprint: for the wave 

 length 0.55m, they quote 3.0 X IQ-^ ergs, (7)" instead of exactly 2.77 X 10"^ 

 If 3 may be taken as a roughly rounded figure for 2.77, however, the order of 

 magnitude is different. In Broca's quotation, another error or misprint is also to 

 be found: 3.6 X 10"^ ergs for 0.75m, instead of 2.56 X IQ-^. 



It may be of interest to compare the tables published by Broca, and Henri 

 and des Bancels with the exact figures of Langley: 



TABLE I. 



2 Langley, (2), pp. 12, 13, 15. 



3 Langley, (2), p. 20. 



* Henri and des Bancels, (7), p. 845. 



