Temperature of Visibility. 551 



Thickness of section =5^ millim. ='002 centim. 

 /.Half „ „ = -001 „ 



7/3 



Flow of heat = -j, where h = conductivity ( = '2), and 6= the 



difference of temperature required, at the ends of the distance 



d ( = -001 centim.), 



•9/9 

 .-. "3=^ whence = O°-OO15. 



The loss of energy per unit area was obtained by a voltmeter- 

 and-ammeter method, which is to be further developed for the 

 Radiation experiments. All the figures are approximate, the 

 smallness of the resulting difference (about yoo~o ° ^0 show- 

 ing that there is here no need for any high degree of accuracy. 



With the surface of the platinum lampblacked the case is 

 different ; but even here an approximate calculation shows 

 that there is still less than 1° difference between the platinum 

 and the surface of the carbon. In a typical case the following 

 results w r ere found: — 



Thickness of layer ='00033 centim. This was determined 

 from a knowledge of the specific gravity of the lampblack. 



Conductivity ='0002. This is the value given by Everett 

 for powdered carbon, and is probably less than the correct 

 value for the lampblack ; but the two must be of about the 

 same order of magnitude. 



Loss of heat per unit area at 500° C. ='5 calorie per 

 second ; 



. .,-00020. . 

 " -00033' •" 



Method of Making the Experiments. 



The first requisite was to get the strip in a perfectly dark 

 enclosure, within which both eyes could be directed towards 

 it without strain. To this end the apparatus was enclosed in 

 a wooden box (blackened within), one end of which was 

 replaced by a black velvet cloth, under which the observer 

 placed his head, and which he could gather round his neck 

 and under his chin so that not a ray of light could penetrate 

 the enclosure. The box was about 48 centim. long, 30 broad, 

 and 22 high, and ordinarily the eyes, in making an observa- 

 tion, would be about 30 centim. from the strip. The other 

 end of the box was provided with a hinged shutter, which was 

 lifted immediately after an observation had been made, for 

 the purpose of noting the temperature of the strip. 



The strip itself was further protected from draughts &c. by 

 means of a piece of brass, bent twice at right angles, and 



