20 An Explanation of the Doctrine of [JtLV, 



A B, CD; let E be a particle of light determined in the direction 

 E G H, perpendicular to the plane, A B, it is evident that the 

 contact of the particle of light with the plane will be in the line of 

 its direction, vw: so likewise at the point of emersion, H, there 

 will be a similar coincidence ; hence there will be no deviation from 

 the right line. 



If the particle of light, I, be determined in the direction K I, 

 forming an angle, K t v, with a perpendicular to the plane, the 

 point it will be the first portion of the particle of light which touches 

 the medium. In this medium it is less resisted than in the medium 

 through which it has just passed : hence the deviation will be 

 towards the perpendicular K O, instead of proceeding in the right 

 line K P, it will describe the line KL; in this case L K P will be 

 the angle of deviation, whatever inclination may be the direction 

 of the particle of light : in the same medium the sines of the 

 angles of deviation and of right direction will bear a constant ratio, 

 i. e. OL will be to O P in an uniform proportion. 



When the particle of light arrives at the other side, C D, the 

 point of contact is at v, and not at L; in that point it is more 

 resisted than iu any other, consequently will be deflected from the 

 perpendicular L R, and will form a corresponding angle of devia- 

 tion, S L M ; and as A B and C D are parallel sides, it is very 

 evident that the emergent particle of light must move in a direction, 

 h M, parallel to its incident direciion, 1 K. 



The same principle readily applies to the direction of a particle 

 of light through any curvilinear media. For this illustration 1 have 

 selected a piano convex and a piano concave. The particle of light 

 in both instances I have supposed as moving in a direction parallel 

 to the axis of each medium ; and in order to show how the particle 

 of light is affected by two refractions, it is arranged so as to enter the 

 curved part of the medium. 



Let A G B, fig. 3, be an hemispheric lens, the centre of which 

 is D j let G D N represent the nxis of the lens ; a particle of light, 

 G, moving in the direction of the axis, will touch the lens in a 

 point coincident with its line of direction : hence no deflection can 

 take place ; the particle of light, E, determined iu the direction, 

 E F, parallel to C G, the point of contact will be ?•; in that point 

 it meets with less resistance than in any other point, consequently 

 the deflection will be towards the perpendicular, which in this case 

 is D F O, instead of proceeding in the direction F P; it will 

 describe the line F T ; and DT:DP = EO, as the sine of the 

 angle of refraction is to that of incidence. 



When the particle of light is arrived at the other side, A B, I is 

 the point of contact; at that point, passing out of glass into air, 

 where it is more resisted, the deviation will be from the perpendi- 

 cular M F, and it will describe the line T N, and where it cuts the 

 axis at N will be the focus. When the curve is uniform, making 

 the allowance for aberration, which from figure is but trifling, all 

 particles of light entering into that medium, the law of resistance 



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