810 LUMINOUS AND DARK HEAT. 



particles of the bodies themselves, and is then said to 

 'be transmitted by conduction. 



Thermal rays have the same velocity as luminous 

 rays, and are subject to the same laws of reflection and 

 refraction. Certain luminous ?&y&, viz., the red, orange, 

 and yellow, are at the same time thermal rays; but 

 many thermal rays do not produce upon our eye the 

 impression of light, and are therefore dark. Such dark 

 thermal rays are less refrangible than the red rays. A 

 warm body which is not luminous emits only dark heat 

 rays; luminous (that is, red, orange, and yellow) heat 

 rays are only emitted by incandescent bodies, but even 

 these emit at the same time considerably more dark 

 than luminous heat rays. 



The sun emits an immense amount of luminous and 

 thermal rays, but much more of the heating effect of 

 the sun is due to the dark than to the luminous thermal 

 rays. Within the small image of the sun produced by a 

 convex lens or a concave mirror, no more heat can be 

 collected than that contained in a bundle of rays of the 

 thickness of the diameter of the lens or mirror, and yet 

 it is sufficient to ignite bodies which are easily combus- 

 tible; while by means of larger appliances for collecting 

 solar rays the most infusible substances have been 

 melted, and some of the least combustible have been 

 ignited. 



Just as luminous rays are allowed to pass only 

 through certain bodies, called transparent, and are 

 stopped by other bodies, called opaque, so are thermal 

 rays only transmitted through certain substances, which 

 are called diathermanous, while all other substances 

 which stop thermal rays are called athermanous. Many 



