ON LIGHT AKD COLOURS. 159 



occasion some more general inquiries founded upon what goes 

 before. This course is dictated by the manifest expediency 

 of first expounding the fundamental principles, and I there- 

 fore begin by respectfully submitting these to the considera- 

 tion of the learned in such matters. 



In the meantime, however, I will mention one inference 

 to be drawn from the foregoing propositions of some interest. 



As it is clear that the disposition varies with the distance, 

 and is inversely as that distance, and as this forms an inherent 

 and essential property of the light itself, what is the result? 

 Plainly this, that the motion of light is quite uniform after 

 flexion, and apparently before also. The flexion produces 

 acceleration but only for an instant. If ss is the space 

 through which the ray moves after entering the sphere of 

 flexion, and v the velocity before it enters that sphere ; it 

 moves after entering with a velocity = *J v* + Z d z, Z being 

 the law of the bending force. Then this is greater than v ; 

 consequently there is an acceleration, though not veiy great ; 



but because y = , if s is the space, t the time, the force of 



s tds sdt a 



acceleration is x 5 ; but y = -- shows that s is 



tds t x 



as t , else y = - - would be impossible ; therefore the accele- 



JC 



s tds s dt 



rating force X ; = 0, and so it is shown there is 



ds t* 



no acceleration after the ray leaves the sphere of flexion. 



