C. GENERAL PHYSICS. 137 



the value of a division of the divided circle maybe easily 

 altered at will, and allows of attaining very great precision 

 with comparatively little labor. Q J3, Jamiari/, 1813, 750. 



THE NATURE OF SUNLIGHT. 



Dr. Draper, of New York, has lately published a summary 

 of the views respecting the activity of the rays of the sun 

 that have long been held by him, and which are now proba- 

 bly almost universally accepted by scientific men, although 

 the elementary text-books on this subject have not yet been 

 divested of the somewhat inaccurate expressions of thirty 

 years ago, which latter also continue to be used by photog- 

 raphers and most practical men. 



According to Dr. Draper, the calorific, luminous, and chem- 

 ical efiects produced by the solar rays are not so many dis- 

 tinct forces or emanations coexistins: in a beam of lisfht, 

 which can be dispersed by a prism, according to a fixed law, 

 over the length of the spectrum ; but are, on the contrary, 

 only tlie various efi*ects of one and the same force acting un- 

 der diiierent conditions and upon different substances. He 

 maintains (1) that the chemical action is not limited to the 

 more refrangible rays, but is equally distributed over the lu- 

 minous and the calorific portions of the spectrum ; (2) that 

 the ray effective in producing chemical or molecular changes 

 in any special substance is determined by the absorptive 

 power of that substance ; (3) that there is also no special lo- 

 calization of the visual or the thermal effects. 



In the case of the silver iodide so generally used by pho- 

 tographers. Draper shows that the more refrangible rays 

 produce an effect contrary to that produced by the less re- 

 frangible. In the case of the bitumens and resins, he shows 

 that a properly prepared film of these is as sensitive to either 

 the ultra red or the ultra violet ravs as the silver iodide is 

 to the latter rays only. 



In the highly important case of the development of the 

 carbonic acid gas found in the atmosphere by the action of 

 sunlight on plants, he shows that this is accomplished by the 

 action of the rays between the orange and the green bands 

 of the spectrum, the maximum effect being in the yellow. 

 The vegetable colors and the colors of flowers are shown to 

 be dependent each upon the chemical action of a correspond- 



