370 LECTURE XXXIX. 



the circumference of a circle, passing through the aperture ; this circle will 

 consequently be white, and it will be surrounded with circles of colours 

 very nearly at equal distances, resembling the stripes produced by diffrac- 

 tion. The analogy between these colours and those of thin plates is by no 

 means so close as Newton supposed it ; since the effect of a plate of any con- 

 siderable thickness must be absolutely lost in white light, after ten or 

 twelve alternations of colours at most, while these effects would require 

 the whole process to remain unaltered, or rather to be renewed, after 

 many thousands or millions of changes. (Plate XXX. Fig. 452.) 



It is presumed, that the accuracy, with which the general law of the 

 interference of light has been shown to be applicable to so great a variety 

 of facts, in circumstances the most dissimilar, will be allowed to establish 

 its validity in the most satisfactory manner. The full confirmation or 

 decided rejection of the theory, by which this law was first suggested, can 

 be expected from time and experience alone ; if it be confuted, our 

 prospects will again be confined within their ancient limits, but if it be 

 fully established, we may expect an ample extension of our views of the 

 operations of nature, by means of our acquaintance with a medium, so 

 powerful and so universal, as that to which the propagation of light must 

 be attributed. 



[The principle of interference which Dr. Young advanced in this lecture 

 and elsewhere, has done much towards establishing the undulatory theory 

 as a true physical theory. This principle explains in the most satisfac- 

 tory way, not only the colours of thin plates, the fringes which accompany 

 shadows, and the like, but more refined and complicated phenomena, such 

 as those produced by placing gratings of different forms before the object 

 glass of a telescope. The simplest form in which the operation of inter- 

 fering light is witnessed, and consequently the most direct mode of com- 

 paring theory witlpi experiment, is to suffer a small pencil of light to fall 

 on a prism of a very large angle (say 179). The two sides of this prism 

 constitute two prisms of an angle of \ each, and serve to bend the same 

 pencil so as to render it virtually two. By receiving the light from these 

 two pencils on any eye piece, it is evident that, in different parts of the 

 field of view, the one will mix with the other in different states of distance 

 from the original focus. In the centre both will have travelled the same 

 distance, and there will be a white bar formed by their mixture. On each 

 side of this, at a certain distance, the one will have travelled further than 

 the other by half the length of a wave. Here the motions of the one will 

 be the reverse of those of the other the one, for instance, tending to raise 

 a particle of the undulating medium, whilst the other tends to depress it, 

 and by the same amount. The result is, that no motion at all ensues, and 

 we are presented with a dark bar : and so on. Moreover, as the lengths 

 of the waves are different for different colours, the next bright bar will not 

 be quite white, the space requisite to allow the one pencil to be in advance 

 of the other l?y a whole undulation (which is equivalent to not being in 

 advance of it at all), being less for the violet rays than for the red. We 

 find, consequently, a coloured fringe ; and as we recede from the centre, 

 the bars become more and more coloured, until the dark of the one alto-] 



