MULTIPLE VISION WITH ONE EYE. 279 



curious optical appearance, noticed by Mr. Hopkinson, in which, by the 

 inflection of light, caused by the threads of a silk handkerchief, a mul- 

 tiple image of a distant lamp was presented. The objections, however, 

 to the explanation by inflection are, that the image always appears sin- 

 gle, if the object be not within the distance of distinct vision ; and 

 that the same multiple image is presented, when the object is seen by 

 reflection, as when we look at a fine line drawn upon paper ; or at a fine 

 needle held in a bright light. In this case, a considerable number of 

 parallel images of the needle may be seen, all equally or nearly equally 

 distinct ; and not coloured. 



Dr. Jurin considers the phenomena to be caused by fits of easy re- 

 fraction and reflection of light. Newton demonstrated, that the rays 

 of light are not, in all parts of their progress, in the same disposition 

 to be transmitted from one transparent medium into another ; and that 

 sometimes a ray, which is transmitted through the surface of the second 

 medium, would be reflected back from that surface, if the ray had a 

 little farther to go before it impinged upon it. This change of dispo- 

 sition in the rays, to be either transmitted by refraction, or to be re- 

 flected by the surface of a transparent medium, he called their jits of 

 easy refraction, and fits of easy reflection; and he showed, that these 

 fits succeed each other alternately at very small intervals in the pro- 

 gress of the rays. Newton does not attempt to explain the origin of 

 these fits, or the cause that produces them ; but it has been suggested, 

 that a tolerable idea of them may be formed by supposing, that each 

 particle of light, after its emanation from a body, revolves round an 

 axis perpendicular to the direction of its motion, and presents alter- 

 nately to the line of its motion an attractive and a repulsive pole, in 

 virtue of which it will be refracted, if the attractive pole be nearest any 

 refracting surface on which it falls ; and reflected, if the repulsive pole 

 be nearest the surface. 



A less scientific notion of the hypothesis has been suggested ; by sup- 

 posing a body with a sharp and a blunt end passing through space, and 

 successively presenting its sharp and blunt ends to the line of its mo- 

 tion. When the sharp 



end encounters any soft Fi s- 125 - 



body it penetrates it ; 

 but when the blunt end 

 encounters the same body, 

 it is reflected or driven 

 back. In applying this 

 explanation to the pheno- 

 menon in question, Dr. 

 Jurin presumes, that the 

 light, in passing through 

 the humours of the eye, 

 experiences these fits of 

 easy refraction and easy 



n j . m , . .,, , J Multiple Vision with One Eye. 



reflection. Inis will be 



elucidated by the marginal figure, Fig. 125. Suppose a number of 



rays of light to proceed from the point A, and to impinge, with dif- 



