THE ANALYSIS OF LIGHT. 345 



visible colored rays of the spectrum, have all the properties of radiant heat 

 coming from bodies of a lower temperature than 800° F. Such heat is much 

 less refrangible than red light ; but if the temperature of the radiating body- 

 be increased, it emits, in addition to the rays previously emitted, others of a 

 higher refrangibility, until at last some few of its rays become as refrangible 

 as the least refrangible rays of light. The body then appears of the same 

 color as the least refrangible rays of light, and is said to be red hot. If it 

 be heated more, it emits, in addition to the red, still more refrangible rays, 

 yIz., orange; then (at a higher temperature) yellow rays are added, and so 

 on, until when the body is white hot, it emits all the colors visible to us; 

 and in some instances (of very intense heat), even the invisible chemical rays, 

 more refrangible than the violet, are emitted, though in less quantity than 

 in the solar rays. Thus light appears to be nothing more than visible heat, 

 and heat invisible light — the constitution of the eye being such that it can 

 perceive one and not the other, in the same way as the ear can appreciate 

 vibrations of sound more rapid than sixteen per second, but not those which 

 are less rapid. 



..^ . . TOS. The study of the chemical principle contained in the 



What curious •' /. 



fact has the rays of solar light has rendered probable the curious fact, that 



study of the ^q substance can be exposed to the sun's ravs without un- 



chcuucal pnn- ^ 



ciple of light dergoing a chemical change ; and from numerous examples it 



*^° ^^ would seem that the changes in the molecular condition of 



bodies which sunlight eflects during the daytime, is made up during the 

 hours of night, when the action is no longer intluencing them. Thus dark- 

 ness appears to be essential to the healthy condition of all organized and un- 

 organized forms of matter. 



Upon what does TliG procGss of forming Daguerreotype and 

 ofphotograpwc otliGr pliotograpliic pictures, depends solely 

 pictures depend? ^p^^ ^j^^ actinic, or cliemical influence of the 

 solar ray. 



The terra "photography," signifying light drawing, which is the general 

 name given to this art, is unfortunate and ill-chosen, for not only does light 

 not exercise any influence in producing the pictures, but it tends to destroy 

 them. 



.^^^ The essential steps of the process of forming a Daguerre» 



•ssentiai steps otvpe picture consist in coating a suitable plate of met;d with 

 ** TT^\ ^'^' some chemical compound easily affected by the action of tha 

 process ? solar ray. Such a coating is usually a compound of the ele- 



mentary body Iodine. The plate is then exposed to the imago 

 formed by the lens of a camera obscura. Relatively, the quantity of light and 

 actinism reflected from any object are the same ; therefore as the light and 

 shadows of the luminous image vary, so will the power of producing change 

 upon the plate vary, and the result will be the production of an image which 

 will be a faithful copy of nature, witli reversed lights and shadows ; the 

 lights darkening the plate, while the shadows preserve it white, or unaltered. 



15* 



