1869.] The Ethereal Hypothesis of Light. 9 



effect upon matter. It will be found described in Professor Tyn- 

 dall's ' Eadiation,'* already quoted, and is one for which we are 

 indebted to the researches of Dr. Draper. By means of a current 

 of electricity, a platinum mre is gradually raised to a state of in- 

 candescence, and after the luminous rays emitted by the wu'e have 

 passed through a prism, the prismatic colours appear one by one as 

 the light becomes more intense, beginning at the red, and ending 

 at the violet end of the spectrum, until from a white light in the 

 wire the whole of the spectrum is obtained upon the screen. 



And now, before we endeavour to glean from these jihenomena 

 what information we are able concerning the action of light, let us 

 try to define what " light " means. Of course, according to the 

 "ethereal hypothesis," it is the vibrations of the atoms of the 

 hypothetical " ether ; " but if we adopted this definition we should 

 be admitting the hypothesis of which we desire to test the accuracy, 

 and should be reasoning from the unknown to the known : this we 

 must of course avoid. 



All observers will agree in regarding light as & force operating 

 upon and causing a motion of matter. It proceeds in a right line 

 and passes freely, and probably unchanged except in degree of 

 intensity, through air, through what we call a vacuum, and, when 

 it falls upon them at certain angles, through other forms of matter 

 which are known as transparent ; but it is also a force, capable 

 when it reaches some forms of matter, (by what means we cannot 

 say), of resolving itself into three or more distinct modes of action, 

 differing in their nature and operation. Sometimes one of these 

 modifications of the force is incapable of producing any perceptible 

 effect upon a particular form of matter, and then it reflects back 

 upon and through the same unknown medium until it reaches 

 the retina, where it produces the effect known as colour. At 

 other times some portion of the force is inoperative when it reaches 

 the surface of the material object, and then its reaction or reflexion 

 causes a combination of colours. In other cases again, chiefly when 

 the force reaches certain forms of matter at a particular angle, no 

 portion of the force is able to affect it, and then we have what is 

 called the reflexion of ordinary light, but there are cases where the 

 whole force is capable of acting upon and through the same form of 

 matter, but whereas it entered it as one whole force, it issues from 

 it as three or more distinct forces or phases of force, and those forces, 

 when they again reach certain forms of matter, reflect upon the 

 retina as a complete " spectrum." 



Of all these eflects of the force " light " we have had examples 



in the phenomena already referred to, and in some of them, as for 



example in the sunstone, we had, in the same object, illustrations of 



nearly all the modifications to whicli the original force is subject, 



* ' Radiatio]!,' pp. 2, .3. 



