b MR. J. SMITH ON THE ORIGIN OP COLOUR 



II. To attempt an explanation of prismatic pheno- 



mena by means of the laws which have been 

 discovered. And 



III. To illustrate my views by experiment. 



9. Before entering on the mechanical experiments by 

 which the subject is to be illustrated, I wish to direct atten- 

 tion to two or three phenomena which were the cause of, 

 first of all, creating dissatisfaction in my mind with the 

 received theories of light, and which I consider, when pro- 

 perly understood, will serve as the basis of the true theory. 

 I shall, however, have a difficulty in making myself under- 

 stood, as I shall be obliged to use a term the adoption of 

 which will not be easily acquiesced in by scientific men as 

 having any physical value until the operation it points out 

 is actually brought under the cognizance of the organs of 

 vision, and even then there may be a difiiculty in defining it. 



The first of the phenomena referred to is the following. 



Experiment I. — Wafer Experiment. 



1 o. A not uncommon experiment is to fix two wafers on 

 a pane of glass or a sheet of paper, about four inches apart, 

 and to look at them with oblique or strained vision until 

 the one is seen to overlie the other, and the two appear 

 to be one. 



In making this experiment a great many years ago, I 

 tried it with variously coloured wafers. A red and dark 

 blue, nearly black, gave in a favourable light a green for 

 the single image. A black and a red gave also a green, 

 sometimes a brown and at other times a purple, the differ- 

 ence depending on the nature of the colours and also on 

 the luminous state of the atmosphere. 



Having never seen this phase of the phenomenon no- 

 ticed — and I searched for some notice of it in vain after 

 my attention had been particularly directed to it — I 



