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368 PHILOSOPHICAL TKANSACTIONS. [aNNO l687 . 



hence a larger image will be projected on the bottom of the eye, and therefore 

 the moon will appear larger. But the moon's imaginary distance and duskiness 

 gradually vanishing as she rises, a different species is hereby introduced into the 

 eye, and consequently she seems gradually less and less, till again she approaches 

 near the horizon. These two opinions, of Gassendus and the Abbe, being so 

 near a-kin, may be considered together. And first I assert, that a wider or 

 narrower aperture of the pupil neither increases nor diminishes the projection 

 on the retina. I know Honoratus Faber, in his Synopsis Optica, endeavours 

 to prove the contrary to this my assertion, and that after this manner. In 

 fig. 5, A B is an object, EF the greater aperture of the pupil, admitting the 

 projection K I on the retina, whereas the lesser aperture C D admits only the 

 projection GH; but GH is less than KI; wherefore a smaller aperture di- 

 minishes the projection. It is surprising that H. Faber, who undertook to write 

 of optics more accurately than all that went before him, should be guilty of so 

 very gross an error; and that the celebrated Gassendus, and the noble Heve- 

 lius should be of the same opinion: for though the aforesaid demonstration 

 hold most certainly true in direct projections, as in a dark room with a 

 plain hole; yet it will not hold in projections made by refraction, as it is in 

 those on the retina in the eye, by means of the crystalline, and other coats and 

 humours of the eye. For let A B, fig. 6, be a remote object, and E F the 

 crystalline at its large aperture, projecting the image I M on the retina. Let 

 then CD be the smaller aperture of the pupil before the crystalline; then the 

 image I M will be projected as large as before; for the cone of rays E A F con- 

 sists partly of the cone of rays CAD, therefore where the former, E A F, is 

 projected, the latter CAD, as being a part of the former, will be projected 

 also. So that no more is affected by this narrow aperture, but only that the 

 sides of the radiating cones are intercepted, and consequently the point I will 

 be affected with less light, but it shall still be in the same place. What is said 

 of that cone, and that point, may be said of all other cones and other points 

 of the object. From hence appears, first, the invalidity of the account given 

 of the moon's appearance by Gassendus; 2. the reason appears, why a teles- 

 cope's greater or smaller aperture makes no difference in the angle it receives: 

 for imagine E F to be an object-glass of a telescope, ^nd the thing is plain. 

 3. It is evident why a greater or less aperture in a telescope should make the 

 dbjects appear brighter or darker, for thereby, more or less rays are admitted 

 for defining the projection of each point. And this is sufficient for a confuta- 

 tion of Gassendus and Faber. But the Abbe superadds to a greater or smaller 

 aperture of the pupil, as a necessary consequence, a greater and less convexity 

 of the crystalline, as also a lengthening and shortening the tube of the eye, 



