VOL. LXXXVI,] PHILOSOPHICAL TRANSACTIONS. 74 7 



positions prefixed to this paper, that the angle of flexion is less than that of in- 

 cidence, when, in the case of inflection, the angle made by the ray and the body 

 is acute, and when in the case of deflection, that angle is obtuse; and when the 

 ray is perpendicular or parallel, the angle of incidence vanishes in both cases. It 

 is evident therefore, that in both these situations of things the ratio of ISOQ to 

 2036, being that of a less to a greater, will not enable us to find the angle of 

 flexion, though it serves very well when the ray before inflection makes an ob- 

 tuse, and before the deflection, an acute angle. I have therefore mentioned the 

 angle made by the bent ray with the incident, which gives a general formula; 

 for let the angle of incidence be i, and that which the bent ray makes with the 

 incident b, then f being the angle of flexion, we have f = b + i ; so that if i 

 = o; F = b; or if the incident makes an obtuse angle with the body, in the 

 case of deflection, and an acute in that of inflection, then f = i — b, and in 

 the remaining case f = i + b. 



These observations enable us to give a very short summary of optical science. 

 When the particles of light pass at a certain distance from any body, a repulsive 

 power drives them off"; at a distance a little less, this power becomes attractive; 

 at a still less distance, it again becomes repulsive; and at the least distance, it 

 becomes attractive as before; always acting in the same direction. These things 

 hold whatever be the direction of the particles; but if, when produced, it 

 passes through the body, then the nearest repulsive force drives the particles 

 back, and the nearest attractive force either transmits them, or turns them out 

 of their course during transmission. Further; the particles differ in their dis- 

 positions to be acted on by this power, in all these varieties of exertion; and 

 those which are most strongly affected by its exertion in one case, are also most 

 strongly affected by that exertion when varied; except in the cases of refraction, 

 of which we before spoke; and these dispositions of the parts are in all the cases 

 in the same harmonical ratio. Lastly, the cause of these different dispositions 

 is the magnitude of the particles being various. 



All that remains now to be done on this part of the subject is to explain one 

 or two phenomena relating to reflexibility. 1 . It has been remarked, that if 

 we look at a candle, or other luminous body, with our eyes almost shut, bright 

 streaks seem to dart upwards and downwards from it. Newton * explains this 

 by refraction through the humours adhering to the eye-lids. Rohault -f- and 

 Mr. Young | ascribe them to reflections. Des Cartes makes them arise from 

 wrinkles on the eye's surface. De la Hire from refraction through the moisture 

 on the eye-lids, as through a concave lens; and Priestley § from inflection through 

 the lashes. The truth of Sir Isaac's explanation is obvious, because the streaks 

 which dart from the top of the luminous body are formed by the under eye-lid, 



* Lect. Opt. sect. 3, ad finem, t Physica, p. 2+9. Clark's ed. 



{ Phil. Trans. 1793. § On Vision, vol. 2. 



5 C 2 



