290 On the Alichamfm of the Eye. 



When I look at a minute lucid point, fuch as the image of a candle in a fmall concave 

 fpeculum, it appears as a radiated ftar, as a crofs, or as an unequal line, and never as a 

 perfect point, unlefs I apply a, concave lens inclined at a proper angle, to correft the un- 

 equal refradlion of my eye. If I bring the point very near, it fpreads into a furface nearly 

 circular, and almofl: equably illuminated, except fome faint lines, nearly in a radiating 

 diredlion. For this purpofe, the befl image is a candle, or a fmall fpeculum, viewed 

 through a minute lens at fome little diflance, or feen by refledlion in a larger lens If any 

 preflure has been applied to the eye, fuch as that of the finger keeping it fluit, the fight is 

 often confufed for a fhort time after the removal of the finger, and the image is in this cafe 

 fpotty or curdled. The radiating lines are probably occafioned by fome flight inequalities 

 In the furface of the lens, which is very fuperficially furrowed in the diredtion of its fibres : 

 the curdled appearance will be explained hereafter. When the point is further removed, 

 the image becomes evidently oval, the vertical diameter being longeft, and the lines a little 

 more diftindl than before, tlie light being (Irongeft in the neighbourhood of the centre ; but 

 immediately at the centre there is a darker fpot, awing to fuch a flight depreffion at the 

 vertex as is often obfervable in examining the lens after death. The fituation of the rays 

 is conftant, though not regular; the moft confpicuous are feven or eight in number; 

 fometimes about twenty fainter ones may-be counted. Removing the point a little further, 

 the Image becomes a ftiort vertical line ; the rays that diverged horizontally being perfectly 

 collcfted, while the vertical rays are (till feparate. In the next ftage, which is the moft 

 perfeft focus, the line fpreads in the middle, and approaches nearly to a fquare, with pro- 

 jefting angles, but is marked with fome darker lines towards the diagonals. The fquare 

 then flattens into a rhombus, and the rhombus into a horizontal line unequally bright. At 

 every greater diftance, the line lengthens, and acquires alfo breadth, by radiations fliooting 

 out from it, but does not become a uniform furface, the central part remaining always con- 

 fiderably brighteft, in confequence of the fame flattening of the vertex which before made 

 it fainter. Some of thefe figures bear a confiderable analogy to the images derived from tke 

 iefra£tion of oblique rays, (Schol. 4. Prop. IV.) and ftill more ftrongly refemWe a com- 

 bination of two of them in oppofite direftions; fo as to leave no doubt, but that both 

 furfaces of the lens are oblique to the vlfual axis, and co-operate in diftorting the focal 

 point. This may alfo be verified, by obferving the image delineated by a common glafs 

 lens, when inclined to the incident rays. (See Plate XV. Fig. 28 — 40.) 



The vifual axis being fixed in any direflion, I can at the fame time fee a luminous ob- 

 jeft placed laterally at a confiderable diftance from it ; but in various direftions the angle is 

 very different. Upwards it extends to 50 degrees, inwards to 60, downwards to 70, and 

 outwards to 90 degrees. Thefe internal limits of the field of view nearly correfpond with 

 the external limits formed by the difl^erent parts of the face, when the eye is diredted 

 forwards and fomewhat downwards, which is its moft natural pofition ; although the in- 

 ternal limits are a little more extenfive than the external ; and both are well calculated for 

 enabling us to perceive the moft readily, fuch objects as are the moft likely to concern us. 

 4 Dr. 



