on the Mechanism of the Eye. 
2 7 
IV. DIOPTRICAL PROPOSITIONS. 
Proposition I. Phenomenon. 
In all refractions, the ratio of the sine of the angle of 
incidence to the sine of the angle of refraction is constant 
(Newton's Opt. I. Ax. 5. Smith's Opt. 13. Wood’s Opt. 24.) 
Scholium 1. We shall call it the ratio of wtowirpi, and 
m 1, n. In refractions out of air into water, m = 4 and n 
= 3, very nearly; out of air into glass, the ratio is nearly that 
of 3 to 2. 
Scholiums. According to Barrow, (Lect. Opt. ii. 4.) Huygens, 
Euler, ( Conject . phys. circa prop, soni et luminis. Opusc. t. ii.) 
and the opinion which I lately submitted to the Royal Society, 
(Phil. Trans, for 1 800. p. 128,) the velocity of light is the greater 
the rarer the medium: according to Newton, (Schol. Prop. 
96. 1 . i. Princip. Prop. 10. p. 3. 1 . ii. Opt.) and the doctrine 
more generally received, the reverse. On both suppositions, it 
is always the same in the same medium, and varies in the ratio 
of the sines of the angles. This circumstance is of use in faci- 
litating the computation of some very complicated refractions. 
Proposition II. Phenomenon. 
If between two refracting mediums, a third medium, termi- 
nated by parallel surfaces, be interposed, the whole refraction 
will remain unchanged. (Newton’s Opt. 1 . i. p. 2. Prop. 3. 
Smith, r. 399. Wood, 105.) 
Corollary. Hence, when the refractions out of two mediums 
into a third ar^ given, the refraction at the common surface of 
these mediums may be thus found. Let the refractions given 
E 2 
