Temperature of Visibility. 555 



General Conclusions. 



(1) That the minimum temperature of visibility is the same 

 for a bright polished metallic surface as for one covered with 

 lampblack, although the intensity of the radiation in the two 

 cases may be different. 



This result may at first be, to some, unexpected, but a little 

 consideration will show that it might have been, a priori, 

 anticipated. For probably temperature governs the highest 

 wave-length from a radiating body, and wave-length governs 

 visibility, at least after an extremely small intensity of radia- 

 tion has been passed *. 



(2) That the visible limit at the red end of the spectrum 

 varies greatly for a normal eye, according to its state of pre- 

 paration ; i. e. according to the intensity of the light in which 

 the observer has been before making the observation. 



I take my own eyes as normal ; they have been tested in 

 the Anthropometrical Laboratory at S. Kensington, and 

 roughly by Captain Abney's method ; and in the figures given 

 above the results are supported by those obtained by the 

 other " cases/' none of whomf are known to have abnormal 

 sight. 



Speaking generally, we may say that a bright light 

 diminishes the sensitiveness of the eye to radiation of low 

 frequency ; that darkness increases it. Or that, as a rule, 

 the eye is less sensitive in the morning than at night. 



(3) That for the less sensitive condition, the minimum tem- 

 perature of visibility for the surface of a solid is about 470° C, 

 but that this may be much reduced by even a few minutes in a 

 dark room. 



(4) That at night, a surface at a temperature of 410° is 

 visible, and that by resting the eyes in complete darkness, this 

 may be reduced to as low as 370° nearly, below which apparently 

 one cannot go, since 10 minutes' rest appears to be almost as 

 efficacious as 3 hours'. 



(5) That different people s eyes {of no special or known de- 

 parture from normality) differ somewhat in their " minimum 

 temperature of visibility ," but probably not to any great extent, 

 if tested under the same conditions as to preparation, Sfc. 



Case G was a somewhat curious one ; the observer was the 

 one exception among those I tried whose red colour-percep- 

 tion is perhaps not quite normal. He arrived at the lowest 



* Langley, "Energy and Vision," Phil. Mag. xxvii. (1889), shows 

 that the amount of energy sufficient to excite vision is immensely less 

 than that radiating from the strip in these experiments, at least in the 

 low-red wave-lengths. 



t With the possible exception of Case G. 



