118 SALTS DECOMPOSED BY LIGHT. 



then, of course, the image thrown on the screen g g was circular ; but if the screen b b 

 was made to receive these rays at an acute angle, then the image was lenticular. 

 Under the last condition, the phenomenon of diffraction is represented in fig. 70, where 

 a a is the screen, b b the lenticular image cast on it; it is of bright white light except at 

 its central part, c, where there is a dark image produced by the interference of the pass- 

 ing rays. 



448. If, in such an arrangement, the chemical rays do not interfere with each other 

 so as to neutralize effects, chemical action should be produced in every part of the 

 image, even including its central part, c ; but if, on the other hand, these rays are obe- 

 dient to the same laws as the rays of light, then, in the central parts of the image, no 

 chemical effects should ensue ; the problem is, therefore, reduced to the finding how 

 any compound, changeable by these rays, will comport itself on the central and peripheral 

 parts of such an image. 



448. In place of the screen g g, a. substitute was used consisting of two thin plates 

 of mica, with a layer of bromide of silver included between them ; these were mounted 

 in a little ivory frame, abed (Jig. 71), just in the manner that objects are usually mount- 

 ed for the use of the microscope, and the lenticular image cast upon the bromide. 

 After an exposure of five minutes, during which care was taken to keep the sun's place 

 perfectly immovable, and also to avoid all local tremour, which might make the image 

 traverse on the bromide, the result was very apparent, being, as represented in Jig. 72, 

 of the natural size ; the peripheral parts being of a deep brown, and the centre yellowish 

 white. Viewed through a lens, the boundary line was not sharp and distinct, but 

 seemed to merge by an insensible gradation into the unaffected part, as in jig. 73. The 

 conclusion to be drawn from this result possesses no common interest ; for the same 

 reasoning which demonstrates that light consists of undulations of an elastic medium, 

 applies here also. 



450. The chemical rays, thus closely attending the luminous rays, and being, like 

 them, subject to the forces bringing about reflection, refraction, and interference, it 

 would become a matter worthy of inquiry to find whether there be any different classes 

 of these rays analogous to the different coloured rays of light, or the unequally refran- 

 gible and absorbable rays of heat. The salts of silver are only one of a class over which 

 the chemical rays exert an action. The following list contains, I believe, all the me- 

 tallic salts at present known, in the constitution of which changes are brought about 

 by exposure to the sun: 



Chloride of manganese, Iodide of mercury, 



Sulpho-cyanate of iron, Chloride of mercury, 



Sulphate of nickel, Bichloride of mercury, 



Carbonate of lead, Chloride of silver, 



Carbonate of nickel, Bromide of silver, 



Nitrate of bismuth, Sulpho-cyanate of silver, 



Chloride of uranium, Nitrate of silver, 



Sulphate of uranium, Bromate of silver, 



Nitrate of uranium, Chloride of gold, 



Chloride of copper, Chloride of osmium and potassium. 



