Observations on Rff) action. 7 



received laws laid down in every elementary treatise on optics ; 

 and I contend that no refraction or bending of the rays can 

 possibly take place at d (see his figure), for tlie rays cd enter 

 the air perpendicular to the plane surface of the water : con- 

 sequently they must pass on without any refraction." Here 

 the Doctor evidently supposes that only a cylinder of rays 

 proceeds from the halt-crown perpendicular to the surface of 

 the water. A number of his other arguments involve also this 

 gratuitous assumption, " When the eye," says he, " is placed 

 immediately over the half-crown looking down into the water, 

 we see the image, not the piece of money, one-fourth nearer to 

 the eye: here there can be no refraction, as the rays coming 

 to the eye must be at right angles to the surface of the water : 

 here there is no angle of incidence; no angle of refraction; 

 no ratio of 3 to 4." Here the Doctor's reasoning is no doubt 

 conclusive, if we admit him his owii principle which he has 

 liere again assumed as an axiom ; for in this case the half- 

 crown could not evidently be all seen at once, imless the pupil 

 of the eye were at least as large as tlie half-crown itself; and 

 taking the other parts of the eye proportional to this size of 

 the pupil, we may safely conclude that no one since the crea- 

 tion has been gifted with such organs of vision. In tl.e ex- 

 periment which he gives with the prism, his alignments also 

 hinge on the same principle. " Having." says he, " placed 

 a sovereign under tlie plane ol' an equilateral jirism, I found 

 ihat two reflected and not reiiacted images were fonned in each 

 plane, as representetl in the following figure. 



a the sovereign placed under the })laue dc 

 of an equilateral }irism, forms an image at a; 

 which image sends images to b i\\u\ /." Ac- 

 cording to the present theory, two images '^ 

 could not possibly be formed by refraction at 

 h andy"; for a being at right angles to the plane d c, the rays 

 should surter no refraction, but jirocced on to the vertex." 

 Here the Doctor would have much obliged his mathematical 

 readers, had he informed them what lie nieant by a ))oint being 

 at right angles to a plane; but from his usual mode oi" rea- 

 soning, we may suppose that he means the cijJinder of rays 

 from the sovereinn rises at right angles to dc. However, that 

 they are not reflected iniages may be made evident by turning 

 the prism round on one ol its angles d or c, ior then the so- 

 vereign will ajipear quite distorted and tinged with the pris- 

 matic colours; whereas it is well known that any object seen 

 by reflection from a plane surface never ajipears to bi' altered 

 in shapo, but alwavs ])resorves its nafin-al lorm. 



Dr. licade bccni*) surprised abo that Sir I.saac NcwU-n was 



not 



