556 The Minimum Temperature of Visibility. 



temperature in the evening without any long rest in the 

 darkness, and, as is seen above, 17 minutes in perfect darkness 

 produced no alteration ; yet, in the morning, he went up to 

 about the usual figure, so that what I had hoped to find an 

 abnormal case turned out approximately ordinary. 



The loss of distinct colour at the low temperatures is very 

 striking ; the appearance to myself, and to most of the 

 observers, has absolutely nothing of red in it, but is like a 

 white mist — the nearest comparison I can make. 



In the morning observations, however, when the strip dis- 

 appeared at from 460° to 470°, the last appearance was 

 distinctly reddish ; and this agrees with one observation noted 

 at night, when after getting the visibility critical-point at 

 about 390° C, the temperature was raised until one could 

 declare for certain that the light looked red : it was then 

 found to be 449°. 



Of course, in all the observations, the luminous area was 

 most distinctly seen by somewhat averting the gaze from it ; 

 generally I found it best to look in the direction of either far 

 upper corner of the enclosure. 



As already mentioned, most of the observers pronounced 

 the appearance at the critical-point to be that of a " whitish 

 mist ;" one, however, thought he saw a slight " lilac tinge " 

 in it ; and " Case Gr " declared it to be decidedly yellow, 

 which is interesting, because to him a red mark on white 

 paper (such as a pip on a card belonging to one of the red 

 suits of a pack) appears yellow, by artificial light at night. 



In one experiment a plate of glass, -J inch thick, and in 

 another a layer of water, ^ inch thick, were inserted between 

 the strip and the eye, without making the slightest difference 

 in the phenomenon ; showing (1) that the point where these 

 substances begin to be more or less opaque to infra-red radia- 

 tion had not been reached ; (2) that the small difference in 

 intensity produced by their insertion had no appreciable 

 effect. This last conclusion is far more strongly borne out 

 by the equality of temperature in the case of the bare 

 metallic and the black surfaces, and indicates that in all the 

 cases it was ivave-length, and not intensity, which was deter- 

 minative of visibility, so disposing of the possible objection 

 that the difference between " morning " and " evening " 

 might be due merely to the state of enlargement of the pupil 

 of the eye, which would naturally be more contracted at the 

 one time than at the other, thus affecting the total amount of 

 radiation falling on the retina. Also, if such an objection 

 were valid ; it would imply that fatigue of the muscles of the 



