ON THE NATURE OF LIGHT AND COLOURS. 363 



seems to be no easy way of supposing a decomposition of any kind. The 

 phenomena of solar phosphor! appear to resemble greatly the sympathetic 

 sounds of musical instruments, which are agitated by other sounds con- 

 veyed to them through the air : it is difficult to understand in what state 

 the corpuscles of light could be retained by these substances so as to be 

 reemitted after a short space of time ; and if it is true that diamonds are 

 often found, which exhibit a red light after having received a violet light 

 only, it seems impossible to explain this property, on the supposition of the 

 retention and subsequent emission of the same corpuscles. 



Tbe phenomena of the aberration of light agree perfectly well with the 

 system of emanation ; and if the ethereal medium, supposed to pervade 

 the earth ^nd its atmosphere, were carried along before it, and partook 

 materially in its motions, these phenomena could not easily be reconciled 

 with the theory of undulation. But there is no kind of necessity for such 

 a supposition : it will not be denied by the advocates of the Newtonian 

 opinion that all material bodies are sufficiently porous to leave a medium 

 pervading them almost absolutely at rest ; and if this be granted, the 

 effects of aberration will appear to be precisely the same in either hypo- 

 thesis. 



The unusual refraction of the Iceland spar has been most accurately 

 and satisfactorily explained by Huygens, on the simple supposition that 

 this crystal possesses the property of transmitting an impulse more rapidly 

 in one direction than in another ; whence he infers that the undulations 

 constituting light must assume a spheroidical instead of a spherical form, 

 and lays down such laws for the direction of its motion, as are incompar- 

 ably more consistent with experiment than any attempts which have been 

 made to accommodate the phenomena to other principles. It is true that 

 nothing has yet been done to assist us in understanding the effects of a 

 subsequent refraction by a second crystal,* unless any person can be satis- 

 fied with the name of polarity assigned by Newton to a property which he 

 attributes to the particles of light, and which he supposes to direct them in 

 the species of refraction which they are to undergo : but on any hypothesis, 

 until we discover the reason why a part of the light is at first refracted in 

 the usual manner, and another part in the unusual manner, we have no 

 right to expect that we should understand how these dispositions are con- 

 tinued or modified, when the process is repeated. 



In order to explain, in the system of emanation, the dispersion of the 

 rays of different colours by means of refraction, it is necessary to suppose 

 that all refractive mediums have an elective attraction, acting more 

 powerfully on the violet rays, in proportion to their mass, than on the red. 

 But an elective attraction of this kind is a property foreign to mechanical 

 philosophy, and when we use the term in chemistry, we only confess our 

 incapacity to assign a mechanical cause for the effect, and refer to an ana- 

 logy with other facts, of which the intimate nature is perfectly unknown 

 to us. It is not indeed very easy to give a demonstrative theory of the 

 dispersion of coloured light upon the supposition of undulatory motion ; 

 but we may derive a very satisfactory illustration from the well known 

 * See additional remarks at the end of this Lecture. 



