298 Lord Kelvin on 



Monte Rosa was due to dust. It is quite possible that this 

 conclusion might be exactly true, and it is fairly probable 

 that it is an approximation to the truth. But on the whole 

 these observations indicate, so far so they can be trusted, the 

 probability of at least as large a value as 10 20 for JV. 



§ 75. All the observations referred to in §§ 57-74 are 

 vitiated by essentially involving the physiological judgment 

 of perception of difference of strengths of two lights of 

 different colours. In looking at two very differently tinted 

 shadows of a pencil side by side, one of them blue or violet 

 cast by a comparatively near candle, the other reddish-yellow 

 cast by a distant brilliantly white incandescent lamp or by a 

 more distant electric aTc-lamp, or by the moon, when practising 

 Rumford's method of photometry, it is quite wonderful to 

 find how unanimous half-a-dozen laboratory students, or even 

 less skilled observers, are in declaring This is the stronger ! 

 or, That is the stronger ! or, Neither is stronger than the 

 other ! When ihe two shadows are declared equally strong, 

 the declaration is that the differently tinted lights from the 

 two shadowed places side by side on the white paper are, 

 according to the physiological perception by the eye, equally 

 strong. But this has no meaning in respect to any definite 

 component parts of the two lights ; and the unanimity, or the 

 greatness of the majority, of the observers declaring it only 

 proves a physiological agreement in the perceptivity of 

 healthy average eyes (from which colour-blind eyes would no 

 doubt differ wildly). Two circular areas of white paper in 

 Sella's observations on Monte Rosa, a circle and a surrounding 

 area of ground glass in Majorana's observations with his own 

 beautiful sky-photometer on Etna, are seen illuminated re- 

 spectively by diminished sunlight of unchanged tint and by 

 light from the blue sky. The sun-lit areas seem reddish- 

 yellow by contrast with the sky-lit areas which are azure blue. 

 What is meant when the two areas differing so splendidly are 

 declared to be equally luminous ? The nearest approach to 

 an answer to this question is given at the end of § 71 above, 

 and is eminently unsatisfactory. The same may be truly 

 said of the dealing with Bouguer's datum in § 57, though 

 the observers on whom Bouguer founded do not seem to 

 have been disturbed by knowledge that there was anything 

 indefinite in what they were trying to define or to find by 

 observation. 



§ 76. To obtain results not vitiated by the imperfection of 

 the physiological judgment described in § 75, Newton's pris- 



