Proceedings of (he Royal Inditution. 405 



certain degree, and suddenly withdrawing it, such spectra would be 

 vividly seen. But one of the most remarkable appearances arose 

 from looking edgeways through the bottles — when the compound 

 colour, the pure colour, and the complementary colours were all seen 

 at the same time : thus, if the red and blue bottles were brought near 

 enough to touch on each side of the nose, the red on the right side, 

 and the blue on the left, yet in looking against a window, so that each 

 eye could perceive the window on the outsides of the bottles, the 

 centre where the colours mixed appeared purple ; and at first the right 

 eye perceived red, and the left blue : but the complementary colour 

 was soon so powerfully excited, that the sky from the window on 

 the right appeared a very blue green, and on the left a red orange |' 

 and this so powerfully, that the experimenter would doubt in which 

 hand he held the red, and which the blue. Intensity of colour thus 

 presented to the eye is soon lessened, and even destroyed, by the 

 complementary spectrum: having, with the flat side of the bottles 

 covered the eye, the blue on the right excites an orange, and the 

 yellow on the left a purple ; remove the bottles, and then with both 

 eyes open, there appears for a moment only a genial reddish tinge 

 upon all objects : but a rapid alternation of opening each eye will 

 present, for some time, all objects tinged with orange by the right 

 eye, and purple with the left ; and the vivid changes which arise 

 from the contrast of colours is strikingly beautiful. 



Ju7ie llth. 

 On the Laws of co-existing Vibrations in Strings and Rods. — As on 

 former occasions on which parts of the science of sound made the 

 subject of the evening's investigations, the philosophy on this occasion 

 belonged to Mr. Wheatstone, the delivery to Mr. Faraday. The 

 general nature of the vibrations of strings and rods producing sounds 

 was first referred to, and the laws which governed them, when the 

 vibrations were of the lowest mode, i. e. when the rods or strings were 

 vibrating as a whole. Then the subdivisions of a string and rod 

 were referred to, in which case nodes vyere produced. Taking the 

 string .iiyst; a8^. the, simplest case, i^ was shewn by experhnent, 

 that it might virtually be subdivided into two or more aliquot parts, 

 each part vibrating with a velocity, increasing as the part was 

 smaller, and the string in all these cases, producmg a cor- 



