(^8 ON THE APPARENT FIGURE OF STAES. 



this appear- them ; it remains to ascertain, for the explanation of the phae- 

 "omenon, how this appearance may be produced, and what 

 part or parts of the organ produce if. 

 Rays caused The lachrymal fluid, by which the cornea is constantly co- 



mal fluid*arZ" ^'^^^^' gives rise to the perception of several rays, when, in 

 of a different winking the eyes, the two lids approach the iris : but these rays 

 *" ■ are essentially different from those here considered, to which 



the name of irradiation has been given. The rays produced by 

 the lachrymal fluid are all perpendicular to the direction of the 

 eye-lids, and are produced only when these are brought very 

 rear together. They are seen sometimes at top only, some- 

 times at bottom, sometimes both at once, j and this according 

 to the position of the eye-lids with respect to the iris : and 

 they are perceived at all distances from the luminous object. 

 Irradiation, on the contrary, is discerned only when the light is 

 at a very great distance, and seen under a very small angle. 

 The separation of the eye-lids has no influence on this pheno- 

 menon : it is always perceived, however wide they are asunder : 

 and lastly the rays are seen in four, five, six, or eight direc- 

 tions, one of which is always parallel to the eye-lids. 



The cause not ^g ^j^g production of this phaenornenon cannot be ascribed 

 in the retina, "^ ' 



or aqueous or to the retina that covers the bottom of the eye, and receives the 

 vitreous hu- jmage ; or to the aqueous and vitreous humours, through which 

 the pencil of light passes ; we have every reason to believe, that 

 it is owing to the action of the cornea, or of the crystalline, or 

 to the action of both conjointly. 

 Effect of the The cornea and crystalline, by the nature of the curvature of 

 cornea and their surface, refract the divergent rays that arrive at the eye, 

 crystalline. amj, cause them to converge to a particular focus. Now it is 

 demonstrable by analysis, and may be verified by experiment, 

 as I have ascertained, that if the surfaces separating mediums 

 be segments of a sphere, the image produced by the rays ema- 

 nating from a luminous object, and received on a plane perpen- 

 dicular to the axis of the pencil is always a circle : but if the 

 Effect of a curved surface, convex toward the least refracting medium, be 



compound srenerated by two different osculatory radii, the image is formed 

 curvature of a ° ■' j ■> a 



refracting sur- of two ellipses, which intersect each other at an angle depend- 

 ^*^^' ingon the position of these two radii. If, therefore, the sur- 



faces of the cornea, or of the crystalline, be not segments of a 

 sphere, this is sufficient to cause the image, formed at the bot- 



ton; 



