IX. EXPERIMENTS AND CALCULATIONS 



RELATIVE TO 



PHYSICAL OPTICS. 



BY 



THOMAS YOUNG, M.D. F.R.S. 



FROM THE PHILOSOPHICAL TRANSACTIONS. 

 Read before the Royal Society, November 24, 1803. 



1. EXPERIMENTAL DEMONSTRATION OF 

 THE GENERAL LAW OF THE INTERFER- 

 ENCE OF LIGHT. 



JLn making some experiments on the fringes 

 of colours accompanying shadows, I have 

 found so simple and so demonstrative a proof 

 of the general law of the interference of two 

 portions of light, which I have already en- 

 deavoured to establish, that I think it right 

 to lay before the Royal Society a short 

 statement of the facts, which appear to me 

 to be thus decisive. The proposilion, on which 

 1 mean to insist at present, is simply this, 

 that fringes of colours are produced by the 

 interference of two portions of light; and I 

 think it will not be denied by the most preju- 

 diced, that the assertion is proved by the 

 experiments I am about to relate, which may 

 be repeated with great ease, whenever the 

 sun shines, and without any other apparatus 

 than is at hand to every one. 



Exper, 1 . 1 made a small hole in a window 



shutter, and covered it with a piece of thick 

 paper, which I perforated with a fine needle. 

 For greater convenience of observation, t 

 placed a small looking glass without the win- 

 dow, shutter, in such a position as to reflect 

 the sun's light, in a direction nearly hori- 

 zontal, upon the opposite wall, and to cause 

 the cone of diverging light to pass over a 

 table, on which were several little screens of 

 card paper. 1 brought into the sunbeam a 

 slip of card, about one thirtieth. of an inch 

 in breadth, and observed its shadow, either 

 on the wall, or on other cards held at dif- 

 ferent distances. Besides the fringes of co- 

 lours on each side of the sh^idow, the shjidow 

 itself was divided by similar parallel fringes, 

 of smaller dimensions, differing in number, 

 according to the distance at which the sha- 

 dow was observed, but leaving the middle of 

 the shadow always white. Now these fringes 

 were the joint effects of the portions of light 

 passing on each side of the slip of card, and 

 inflected, or rather diflracted, into the sha- 

 dow. For, a little screen being placed either 



