of the Interference of Homogeneous Light, 83 



light, which proceeds as from the points a and b 9 is always on 

 the line mn, and the other bands are symmetrical on each side 

 of it. After refraction by the prism, the central band is no 

 longer on the new line m' n\ but follows another direction si- 

 milar to p q, which, when we come to examine this subject 

 mathematically, we shall find leads us to very important con- 

 clusions. 



As I shall have to consider the points at which interference 

 should take place according to the Newtonian hypothesis, — that 

 light moves with greater velocity in passing through bodies, 

 in the direct ratio of their refractive indices, — I shall first show 

 how we may expect interference to take place, on the theory 

 that the phenomena of light arise from the effects of a subtile 

 matter which is emitted by luminous bodies. The discovery 

 of interference renders certain conditions now essential in 

 every theory of light. These are: 1st, That luminiferous sur- 

 faces expand around each luminous point, which expanding 

 surfaces tend to as near a spherical form as the circumstances 

 of the luminous body permit. 



2nd, That these luminiferous surfaces, or shells, succeed 

 each other at regular intervals, which differ for the different co- 

 lours of the spectrum, and are for each colour exactly the 

 double of Newton's fits of easy reflection and transmission. 



Those who admit the material theory of light, generally 

 allow that light and heat are mutually convertible ; and many 

 facts may be adduced which confirm this view. Now heat 

 perpetually radiates without causing any impression of light 

 in the eye, and we must suppose that interfering rays of light 

 produce an effect on the organ of vision similar to radiant 

 heat; so that when two rays arrive on the same path in juxta- 

 position, they cease, either by their combined bulks being too 

 large, or by some other property unknown to us, to create 

 that sensation which we call light. That the eye has only a 

 very limited power of appreciating the impressions which may 

 be conveyed to it, the advocates of every theory must allow. 



There is a .fundamental difference, however, between this 

 way of viewing interference and that adopted on the undulatory 

 theory ; for on this view the rays would neutralize each other's 

 effect, while on the undulatory theory they should strengthen 

 it, and vice versa ; that is, there should be a difference in the 

 effects equal to the half of an intervening space, or half the 

 breadth of an undulation. In one theory the effect of bright- 

 ness should ensue where darkness should arise according to 

 the other. The unfortunate half undulation which has con- 

 tinually to be asked for by those who adopt the undulatory 

 theory of light, to make their theoretical deductions agree 



M2 



