THEORY OF llGHT AJID COLOURS. 173 



defcribe a circle concentric with the furfaces of the inflecling 

 atmofphere, then the angle defcribed by the ray during its 

 paiTage through the atmofphere, will always be to the angle 

 fubtended by the line cut off by this circle from the incident 

 ray produced, in the ratio of r to r — i ; and the time fpent 

 in this palfage, will be in the fame ratio to the time that would 

 have been fpent in defcribing this intercepted portion with the 



i 

 initial velocity. For 3/, being equal to s x r > is the fine 

 of the inclination of the incident ray to the radius, where it 

 meets this circle; therefore by the proportion quoted, the an- 

 gle defcribed is in a given ratio to the angle at the centre, 



1 



which is the difference of the inclinations. Making x x 7* 



s 

 •or — radius, the fi-ne, inilead of y } becomes .?, and the cofine 



VOL. — ss, or —y/l -~yy, and, when y = .?.?, x / 1 — as; 

 yy y 



therefore the line intercepted is to the difference of the fluents 

 as r to r — 1 . (See alfo Young's Syllabus, Art. 372.) 



PROPOSITION IX. 



-Radiant Light confijls in Undulations of the luminiferous Ether . Conclusion. Ra- 

 This propofiiion is (he general conclufion from all the pre- SS'ia^StaET 

 ceding ; and it is conceived that they confpire to prove it in as tions of the 

 fatisfa&ory a manner as can poflibly be expecled from the na- ether * 

 ture of the fubjeft. It is clearly granted by Newton, that 

 there are undulations, yet he denies that they conftitute light ; 

 but it is -mown in the three firft Corollaries of the laft Propofi- 

 iion,. that all cafes of the increafe or diminution of light are 

 referable to an increafe or diminution of fuch undulations, and 

 that all the affections to which the undulations would be liable, 

 are diftindtly vifible in the phenomena of light ; it may there- 

 fore be very logically inferred, that the undulations are light. 



A few detached remarks will ferve to obviate fome objec- 

 tions which may be raifed againft this theory. 



1 . Newton has advanced the fingular refraction of the Ice- Remarks. New- 

 land cryftal, as an argument that the particles of light mull be F on with ° ut g' ,v ' 

 projected corpufcles ; fince he thinks it probable that the dif- j e a e d the law of 

 ferent fides of thefe particles muft be differently attraded by Huygens in ex- 

 the cryftal, and fince Huygcns has confeflbd his inability to Jhenomeiuof 



account Iceland cryftal. 



