302 PHILOSOPHICAL TRANSACTIONS. [ANNO 1728. 



But this is only cavilling about words; for if the ray of light, which moving 

 in a dense medium, falls obliquely on the surface common to that and a 

 rarer medium, be turned back again in the dense medium, so as to make the 

 angle in which it returns from the said surface, equal to that in which it came 

 to it ; this return of the ray may properly be called a reflection, whether the 

 ray be turned back at the point of the incidence in the surface, or be carried 

 about the point of incidence in a small curve, whose consideration may be 

 omitted in tracing the way of a ray of light in its passage, for making of 

 optical machines. Whoever reads the 8th Prop, of the 2d part, Book 2, of 

 Sir Isaac's Optics, may very easily find that he was not ignorant of the turning 

 back of the ray under the surface of the glass before it returned into it: and 

 though the reflection in that case be not made by impinging on the solid parts 

 of the glass, yet it is owing to them that the light, acted upon at a distance, 

 is turned up again, as has been shown by several of the experiments above- 

 mentioned. 



Now let us see how Rizzetti's account of the new immersion agrees with 

 phaenomena. 



Let all above the line pp, fig. 19, be a dense medium, as glass; and all 

 below it a rare medium, as air ; abcd is a beam of light insensible in thickness, 

 but of some breadth, whose rays cohere to one another, and whose section or 

 first line is bc. If the medium in which bc is, did not change, bc would 

 move parallel to itself in the lines Ba and cd; but as the end c of the line bc 

 comes out into a rare medium, which being of less resistance to light (for so 

 he supposes), the point c moving with more facility than the point b, describes 

 the curve cfh, while b moving in the dense medium with more difiiculty, de- 

 scribes the lesser curve beg; then the point c, being got to h, is re-immersed, 

 and the line bc, being got to hg, goes on in the direction hk gl parallel to 

 itself, drawing the beam after it in a rectilinear direction, after part of it has 

 been bent within the glass, and part of it without. 



Now if this be true, and pp^r be a prism, Dr. D. asks what becomes of the 

 line at ef, which unites the rays of the beam about the point of incidence x, 

 when water is brought to touch the surface pp, as at ab, fig. 17 ? If it be said 

 that water making a great resistance, though not so great as glass, the curve 

 BEG deviates so little from the line Ba, that the point e comes below i, and the 

 beam is wholly refracted ; he asks whence comes the faint image at k? If it be 

 answered, that some part ei of the line ef, fig. ig, is turned up to the eye 

 at E, fig. 17 ; what becomes of the lateral cohesion of light on which Rizzetti 

 founds his chief proposition, and from which he draws his consequences? 



It would be tedious, as well as useless, to be particular in showing all Riz- 



