VOL. XVI.] JHILOSOPHICAL TRANSACTIONS. 307 



appear to subtend a greater arch, when in the horizon, than when elevated, 

 yet she appears under the same angle : and all this without refraction. Now 

 the geometry of this figure is most certainly true and demonstrable; but it clears 

 up no more our present difficulty, than if nothing at all had been said: for the 

 philosopher has here made a figure of his own, and from thence he argues as 

 confidently, as if nature would accommodate herself to his scheme, and he not 

 obliged to accommodate his scheme to nature; for here he has made the circle 

 GF, representing the earth, very large in proportion to the circle AD; and 

 then indeed, taking the point F in the earth's surface, and by lines from thence 

 dividing the angle AFD into a number of equal parts, the intercepted arches 

 A B, B C, CD will be unequal. But if he had considered that the earth is, as 

 it were, a point in respect of the sphere of the fixed stars; nay, the very annual 

 orbit of the earth itself is almost, if not altogether imperceptible, he would 

 have found that the lines FB, F C, F D, must be all conceived as drawn from 

 the point G; and then equal angles will intercept equal arches, and equal arches 

 equal angles; and so it happens, at least as to sense, to the eye on the surface 

 of the earth. So that his drawing his lines so far from G as F is, and to another 

 concentric circle so near as A D, deceived him in this point. 



The famous Gassendus has written 4 large epistles on this subject ; the sub- 

 stance of all which is, that the moon when near the horizon, and viewed through 

 a more foggy air, casts a weaker light, and consequently forces not the eye so 

 much as when brighter; and therefore the pupil enlarges itself the more, 

 thereby transmitting a larger projection on the retina. In this opinion I find 

 he is not alone; for in the Journals des Sqavans, a French Abbe follows this 

 sentiment of Gassendus; with this addition, that this contracting and enlarging 

 of the pupil causes a different shape in the eye; an open pupil making the 

 crystalline flatter, and the eye longer: but a narrower pupil shortening the 

 eye, and making the crystalline more convex ; the first attends our looking at 

 objects which are remote, or which we think so; the latter accompanies the 

 viewing objects near at hand. Likewise an open pupil and flat crystalline attend 

 objects of a gentler light, while objects of more forcible rays require a greater 

 convexity, and a narrower pupil. From these positions the Abbe endeavours 

 to account for the phenomenon in the following manner. When the moon is 

 near the horizon, by comparison here, with interposed objects, we are apt to 

 imagine her much farther from us, than when more elevated ; and therefore, 

 says he, we dispose our eyes as for viewing the object farther from us; that is, 

 we enlarge the pupil a little, and thereby make the crystalline flatter; moreover, 

 the duskiness ©f the moon, in that position, does not so much strain the 

 sight; and consequently the pupil will be larger, and the crystalline flatter: 



