630 



ON THE THEORY OF LIGHT AND COLOURS. 



jected corpuscles ; since he thinks it proba- 

 ble that the clifFerent sides of these particles 

 are diRercntly attracted by the crystal, and 

 since Hiiygens has confessed his inability to 

 account in a satisfactory manner . for all the 

 phenomena. But contrariiy to what might 

 have been expected from Newton's usual 

 accuracy and candour, he has laid down a 

 new law for the refraction, without giving a 

 reason for rejecting that of Huygens, which 

 Mr. Haliy has found to be more accurate 

 than Newton's ; and, without attempting to 

 deduce from his own system any explana- 

 tion of the more universal and striking ef- 

 fects of doubling spars, he has omitted to 

 observe, that Huygens's most elegant and in- 

 genious theory perfectly accords with these 

 general effects, in all particulars, and of 

 course derives from them additional preten- 

 sions to truth : this he omits, in order to point 

 out a difficulty, for which only a verbal so- 

 lution can be found in his own theory, and 

 which will probably long remain unexplained 

 by any other. 



2. Mr. Michell has made some expe- 

 riments, which appear to show that the 

 rays of light have an actual momentiun, by 

 means of which, a motion is produced when 

 they fall on a thin plate of copper delicately 

 suspended. (Priestley's Optics.) But, tak- 

 ing for granted the exact peipendiculaiity of 

 the plate, and the absence of any ascending 

 current of air, yet since, in every such expe- 

 riment, a greater quantity of heat must be 

 communicated to the air, at the surface on 

 ■which the light falls, than at the opposite 

 surface, the excess of expansion must neces- 

 sarily produce an excess of pressure on the 

 first surface, and a very perceptible recession 

 «f the plate in the direction of the light. 



Mr. Bennet has repeated the experiment, 

 with a much more sensible apparatus, and 

 also in the absence of air ; and very justly in-, 

 fers, from its total failure, an argument in fa-i 

 vour of the uridulatory system of light. (Phil. 

 Trans. 1792. 87.) for, granting the utmost 

 imaginable subtility of the corpuscles of 

 light, their effects might naturally be ex- 

 pected to bear some proportion, to the effects 

 of the much less rapid motions of the electrical 

 fluid, which are so easily perceptible. 



3. There are some phenomena of the light 

 of solar phosphori, which at first sight might 

 seem to favour the corpuscular system; for 

 instance, its remaining many months as if 

 in a latent state, and its subsequent reemis- 

 sion by the action of heat. But, on further 

 consideration, there is no difficulty in sup- 

 posing the particles of the phosphori, which 

 have been made to vibrate by the action of 

 light, to have this action abruptly suspended 

 by the intervention of cold, whether as con- 

 tracting the bulk of the substance or other- 

 wise ; and again, after the restraint is re- 

 moved, to proceed in their motion, as a 

 spring would do, which had been held fast 

 for a time, in an intermediate stage of its vi- 

 bration ; nor is it impossible that heat itself 

 may, in some circumstances, become in a 

 similar manner latent. (Nicholson's Journal- 

 11- 399-) But the affections of heat may, 

 perhaps, hereafter be rendered more intelli- 

 gible to us ; at present, it seems highly pro- 

 bable, that light differs from heat only in the 

 frequency of its undulations or vibrations ; 

 those undulations which are within certain 

 limits, with respect to frequency, being ca- 

 pable of affecting the o])tic nerve, and con- 

 stituting light; and those which are slower, 

 and probably stronger, constituting heat 



