356 Observatiotis on the Visibility of the Retina ; 



with peculiar pi'ecision, their most minute ramifications being 

 rendered visible. When in this state, if the light is suddenly 

 intercepted by moving the lens, the spectrum changes from 

 dark to bright previous to its disappearance. The vessels 

 near the flame have occasionally been thought to exhibit an 

 effect of light and shade when viewed in this way, or even 

 without the lens, if the distance of the candle from the eye is 

 very small ; the side nearest the flame exhibiting a faint de- 

 gree of reddish illumination : this appearance, however, was 

 rather uncertain. 



The foregoing observations relate solely to the ramifications 

 of the blood-vessels; but the most remarkable facts are those 

 connected with the appearance of the foramen centrale. When 

 the flame is situated very obliquely with respect to the axis of 

 vision, a space is perceived in the centre of the field of view 

 across which no ramifications extend, though a great num- 

 ber of minute ones diverge from it, especially in the right eye, 

 the distribution of the vessels in the two eyes being very dis- 

 similar. As the flame is made to approach, in any direction, 

 to this vacant space, a dark crescent becomes visible within 

 it, bounded on the convex side by a fine line of light. As the 

 flame approximates to this crescent, it increases rapidly in 

 breadth in the centre, becomes of a semilunar form, and could 

 it be seen in apposition with the flame, would apparently be 

 converted into a dark circle. By examining this phasnomenon 

 on every side, and especially by causing the flame to describe 

 a circle round the centre of the field of view, in which case 

 the crescent appears to describe a similar but opposite circle 

 round the same point, it is rendered perfectly evident that this 

 crescent is a shadow upon the interior declivity and bottom of 

 the foramen centrale, which follows in every position the laws 

 of the shadow in a very shallow cup or cavity illuminated by 

 an oblique light. It is everywhere accompanied by a narrow 

 ring at its external edge, more luminous than the surrounding 

 space, precisely as though the depression were encompassed, 

 like the cavities on the lunar surface, by an elevated rim ; but 

 no traces can anywhere be perceived of a shadow cast by this 

 apparent rim outwards, on the side which is furthest from the 

 light, nor can the rim itself be discerned beyond the extent 

 of the dark crescent. The shadow is always better defined on 

 its convex than on its concave side. On the side nearest to 

 the nose, in the apparent position of the cavity, which, how- 

 ever, is unquestionably inverted, both the shadow and the rim 

 are much more conspicuous than in the opposite direction ; they 

 may, however, be satisfactorily traced all round. The foramen 

 centrale, thus distinctly rendered visible, is elliptical in a hori- 



