PHYSICAL OPTIC9. 



the air in either of the balls is made to exceed that of the 

 other, the increafed elalHcity caufes the bubble to move to- 

 wards the colder ball. 



By a fimple contrivance of Aiding boards, the hot bodies 

 were moved by rack-work and a winch to any diftance from 

 the thermofcope without the attention of the obferver being 

 taken off from the bubble in the tube, and the diftances were 

 alfo fliewn by a graduated fcale and nom'us. Fig, 3. Plate I. 

 fliews one of thefe bodies. It is a metallic cylinder having its 

 bafe,[which is to be prefented to the thermofcope, vertical, and 

 its neck obliquely placed for the purpofe of introducing the 

 hot Water, and alfo a thermometer for (hewing its temperature 

 at any required time. 



The experhnents and obfervations witich covfiitute the remainder 

 of this 7Jiemoir will be given in our next. 



XIII. 



Experiments and Calculations relative to phyjical Optics. By 

 Thomas Young, M. D. F. R. S. From the Philofophical 

 TranfaBions for 1804. 



I, EXPERIMENTAL DEMONSTRATION OF THE GENERAL 

 LAW OF THE INTERFERENCE OF LIGHT. 



In making fome experiments on the fringes of colours accom- General laws of 



panying (hadows, I liave found fo fimple and fo demonfirative ^^ jigj^/ ^^^^j 



a proof of the general law of the interference of two portions in the produ<ftioB 



of light, which I have already endeavoured to efiablifli, that I °^i^"s!" °^ 



think it right to lay before the Royal Society, a fhort (iatement 



of the fa6ls which appear to me fo decifive. The propofition 



on which I mean to infift at prefent, is (imply 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 moft prejudiced, 



that the aHTertion is proved by the experiments I am about to 



relate, which may be repeated with great eafe, whenever the 



fun fliines, and without any other apparatus than is at hand to 



every one. 



Exper. 1. I made a fmall hole in a window-fhutter, and J^^P' V Fringes 

 covered it with apiece of thick paper, which I perforated with fhadow"oflilip 



a fine 0^ ^^^ ^^ P>^o* 



