168 



fimilar to that of 

 the ftriated fur* 

 faces; namely 

 colours, 



which will be 

 tranfmitted and 

 reflected by- 

 turns if t e 

 thicknefs gra- 

 dually vary, as 

 in the Newto- 

 nian rings. 



Breadth and 

 duration of the 

 colorific undula 

 tions deter- 

 mined, 



and compared 

 with thofe of 

 found. 



THEORY OF LIGHT AND COLOURS. 



agrees exactly with Newton's experiments ; for the correction 

 is perfectly inconiiderable. 



Let the medium between the furfaces be rarer than the fur- 

 rounding mediums ; then the irrfpulfe reflected at the fecond 

 furface, meeting a fubfequent undulation at the firft, will ren- 

 der the particles of the rarer medium capable of wholly flop- 

 ping the motion of the denfer, and deftroying the reflection, 

 (prop, iv.) while they rhemfeives will be more ftrongly 

 propelled than if they had been at reft ; and the tranfmitted 

 light will be increafed. So that the colours by reflection will 

 be deftroyed, and thole by tranfmiflion rendered more vivid, 

 when the double thickneffes, or intervals of retardation, are 

 any multiples of the whole breadths of the undulations ; and, 

 at intermediate thickneffes the effects will be reverfed ; accord- 

 ing to the Newtonian obfervations. 



If the fame proportions be found to hold good with refpect 

 to thin plates of a denfer medium, which is indeed not impro- 

 bable, it will be neceffary to adopt the corrected demonltra- 

 tion of prop. iv. but, at any rate, if a thin plate be inter- 

 pofed between a rarer and a denfer medium, the colours by 

 reflection and tranfmiflion may be expected to change places. 

 From Newton's meafures of the thickneffes reflecting the 

 . different colours, the breadth and duration of their refpective 

 undulations may be very accurately determined ; although it is 

 not improbable, that when the glaffes approach very near, the 

 atmofphere of ether may produce fome little irregularity. The 

 whole vifible fpectrum appears to be comprifed within the 

 ratio of three to five, or a major fixth in mufic j and the un- 

 dulations of red, yellow, and blue, to be related in magni- 

 tude as the numbers 8, 7, and 6 ; fo that the interval from red 

 to blue is a fourth. The abfolute frequency expreffed in num- 

 bers is too great to be diftinctly conceived, but it may be bet- 

 ter imagined by a comparifon with found. If a chord found- 

 ing the tenor c, could be continually bifected 40 times, and 

 fhould then vibrate, it would afford a yellow green light : 



41 40 



this bring denoted by c, the extreme red would be a, and the 



4. 1 

 blue d. The abfolute length and frequency of each vibration 



is expreffed in the table; fuppofmg light- to travel in 8-J mi- 

 nutes .500,000,000,000 ket 



Colours. 



