p. LECOMTE DU NOtJY 763 



As our figures are in good accord with those of the best authors, 

 within less than 10 per cent, we may assume that this is the upper 

 limit of error. This gives a total of 20 per cent possible error, which 

 is beyond the possibiHty of detection by the eye in the minimum 

 visibile, as stated before. As some of the individual data differ by 

 more than 100 per cent, the data can only be considered as rehab le 

 in the conditions of the experiments, within about 120 to 150 per 

 cent. This is about the order of magnitude of the differences be- 

 tween the experimental data given by Langley. 



Quantum Theory. — We can roughly express the minimum visibile 

 in function of quanta of energy. For the mean radiation 0.55 n, 

 the period of the atom is 5.76 X 10" per second. The minimum 

 of energy perceived is approximately equal to 1.9 X lO^^^ ergs per 

 second, (taking 3.8 X 10"^^ as the value of the minimum for total 

 radiation, and roughly 5 per cent as belonging to the visible spectrum) . 

 Hence, 



;.^ — = 3.3 X 10-27 

 5.75X10'" 



As Planck's universal constant // = 6.5 X 10~", the figure found 

 is satisfactory as far as the order of magnitude is concerned, but it 

 would mean that only one-half quantum per second would be suffi- 

 cient to cause the luminous sensation ; as we have dealt with an area of 

 1 

 r^ of a square millimeter on the retina, it would indicate that 



the destruction of one molecule every 2 seconds on such an area 

 would be sufficient to produce an impression of light. 



CONCLUSION. 



A method was devised for measuring the minimum visibile in 

 different parts of the spectrum, as done by Langley in 1888. 



The results are generally in good agreement with those given by 

 this author, although not as close on both sides of the wave length 

 0.55 m; this may be due partly to the use of a rock salt prism, to the 

 fact that the minimum was determined by looking at a beam of 

 diffused transmitted, instead of diffused reflected light, and also to 

 the fact that Langley experimented with the sun, through the earth's 

 atmosphere, and had to take into account the thickness of the atmos- 



