185l] WRITINGS OP JOSEPH HENRY. 299 



immense velocity, but subject to mechanical laws. The in- 

 ference from this assumption is, that meeting obliquely a 

 reflecting surface the atoms will rebound as would a perfectly 

 elastic ball, making the angle of incidence equal to the angle 

 of reflection. This fact being established by experiment, all 

 the phenomena of reflected light are deduced mathematically 

 as mechanical consequences from the primary assumption. 

 Again, it is discovered that a ray of light, in entering 

 obliquely a new medium, changes its direction; and this is 

 readily explained by adding to the previous hypothesis the 

 second condition, that the atoms of light, like all other matter, 

 are subject to attraction, and that they are, in consequence 

 of this, accelerated or retarded in velocity at the moment of 

 entering the new medium. From this assumption readily 

 flows the law of the permanency of the ratio of the sine of 

 the angle of refraction to that of incidence. 



In the progress of discovery it is further found that a ray 

 of light is separated into different colors; and in order to 

 explain this agreeably to the same analogies, we are obliged 

 to admit that there are different kinds of atoms of light, with 

 diff'erent properties, and moving with different velocities. 

 Further, it is discovered that light, in passing by the edges 

 of different bodies, produces fringes and other phenomena 

 known by the name of diff'raction. To explain these another 

 supplementary hypothesis must be added, namel)'', that the 

 atoms of light are alternately attracted and repelled by the va- 

 riation in their distance from the solid body near which they 

 pass. Another class of phenomena, denominated by New- 

 ton " fits of easy refraction and easy reflection," induce the 

 assumption that the atoms of light are not homogeneous in 

 property on all sides, but that each possesses an attracting 

 and repelling pole; and that in their passage through space 

 they are constantly revolving on axes perpendicular to the 

 line joining their poles. Again, the discovery of Malus 

 requires another supplementary hypothesis in order to a 

 mechanical conception of the phenomena first observed by 

 him. To explain these we must admit that the atoms of 

 light possess different properties on different sides, in addition 



