1814.] Explanation of the Doctrine of Refraction. 19 



Article III. 



An Explanation of the Doctrine of Refraction on Mechanical 

 Principles. By C. H. Wilkinson, M. D. 



(To Dr. Thomson.) 



DEAR SIR, Kingston House, Bath, May 5, 1S14. 



Having for many years, in the annual Course of Lectures I 

 deliver in this city, been accustomed to attempt the explanation of 

 those changes in the condition of inanimate substances, which have 

 been referred to attractive and repulsive influences, to causes purely 

 mechanical; if these conjectures should be worthy of insertion in 

 your Annals of Philosophy, I shall be induced to trouble you with 

 my further remarks. I am, dear Sir, 



Yours most respectfully, 



C. H. Wilkinson. 



Suppose A M H N, Plate XX. fig. 1, to represent a section of 

 a reservoir of water ; A M, the horizontal plane ; let B represent 

 a ball directed in the line, C E, perpendicular to the plane, AM; 

 the point, D, where it will touch the plane, will be in the line of 

 direction C E, entering into another medium ; all its forces will 

 conspire in the same line of direction to overcome the resistance, 

 the velocity will be diminished in the ratio of that resistance, but 

 the direction will be preserved uniform in the perpendicular line, 

 CDE. 



If the ball should be projected in the oblique direction, GFR, 

 forming an angle, G R M, with the plane, A M, the ball will be 

 in contact with the resisting medium at the point, O, forming the 

 acute angle, O F R, the compliment of the angle of inclination of 

 the ball with the horizon, or with the surface of the fluid, A M. 

 The ball being more resisted at O than in any other part it becomes 

 deflected from the direction G R S, and proceeds in the line R H, 

 so that the angle S R H is the angle of deviation, induced by the 

 resisting medium. 



At whatever angle the ball be directed, provided it penetrates the 

 medium, H T and S T will preserve a constant ratio. 



It will be equally evident, that if the ball were propelled so as to 

 pa«s out of o.ie medium into another less resisting, that the angle 

 of deviation would he reversed, so that the sine of the refractive 

 angle would be less than the sine of the incident angle. 



In the above example it is evident that the resistance will be in 

 prop o r t ion to the density of the fluid : such will noways o| erate as 

 an objection to the application of the same principle, CO acc< nut for 

 that deviation from the right line, of a [-article ot light, when de- 

 tennined out of one medium into another. 



Let A Ji C D, lig. 2, represent any m diurn with parallel sides, 



* 2 



