ON THE NATUEE OF LIGHT AND COLOURS. 471 



rainbows, nearly in the same proportions as the external fringes of a shadow, 

 or the rings seen in a concave plate. (Plate XXX. Fig. 451.) 



The last species of the colours of double lights, which it will be necessary 

 to notice, constitutes those which have been denominated, from Newton's 

 experiments, the colours of thick plates, but which may be called, with 

 more propriety, the colours of concave mirrors. The anterior surface of a 

 mirror of glass, or any other transparent surface placed before a speculum of 

 metal, dissipates irregularly in every direction two portions of light, one 

 before, and the other after its reflection. When the light falls obliquely on 

 the mirror, being admitted through an aperture near the centre of its curvature, it 

 is easy to show, from the laws of reflection, that the two portions, thus dissipated* 

 will conspire in their effects, throughout the circumferenceof 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 diffraction. 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 considerable 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 inter- 

 ference of light has been shown to be applicable to so great a variety of foots, 

 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. 



