18G 



Subdivifion of 

 the l ; ght of a 

 candle explain. 



DR. YOUNG 3 ACCOUNT OF SOME CASES. 



dimenfions deduced from the phenomena of thin plates. Hence 

 it happens, that when a line of- the light proceeding to form an 

 image of the rings of colours of thin plates, is intercepted by a 

 prifm, and an actual picture is formed, refembling the fcale de- 

 lineated by Newton from theory, for eftimating the colours of 

 particles of given dimenfions, the oblique fpeclrums, formed by 

 the different colours of each feries, are not ftraight, but curved, 

 the lateral refraction of the prifm feparating. the violet end 

 more widely than the red. The thicknefs corresponding to the 

 extreme red, the line of yellow, bright green, bright blue, and 

 extreme violet, I found to be inversely as the numbers 27, 30, 

 35, 40, and 45, refpectively. In confequence of Dr. Wollaiton's 

 correction of the defcription of the prifmatic Spectrum, com- 

 pared with thefe observations, it becomes necelfary to modify 

 the fuppofition that I advanced in the laft Bakerian ledture, 

 respecting the proportions of the Sympathetic fibres of the 

 retina ; fubftituting red, green, and violet, for red, yellow, 

 and blue, and the numbers 7, 6, and 5, for 8, 7, and 6. 



The fame prifmatic analyfis of the colours of thin plates, 

 appears to furnifli a Satisfactory explanation of the Subdivision 

 of the light of the lower part of a candle : for, in fact, the light 

 tranfmitted through every part of a thin plate, is divided in a 

 fimilar manner into diftinct portions, increafing in number with 

 the thicknefs of the plate, until they become too minute to be 

 vifible. At the thickneSs correfponding to the ninth or tenth 

 portion of red light, the number of portions of different colours 

 is five ; and their proportions, as exhibited by refraction, are 

 nearly the fame as in the light of a candle, the violet being the 

 broadelt. We have only to fuppofe each particle of tallow to 

 be, at its firlt evaporation, of Such dimenfions as to produce the 

 fame effect as the thin plate of air at this point, where it is 

 about to^o^ of an inch in thicknefs, and to reflect, or perhaps 

 rather to tranfmit, the mixed light produced by the incipient 

 combuStion around it, and we (hall have a light completely re- 

 fembling that which Dr. Wollafton has obferved. There 

 appears to be alfo a fine line of ltrong yellow light, Separate 

 from the general fpectrum, principally derived from the moil 

 Superficial combuftion at the margin of the flame, and increaf- 

 ing in quantity as the flame afcends. Similar circumftances 

 might undoubtedly be found in other cafes of the production 

 or modification of light ; and experiments upon this Subject 



might 



