xvii'h Remarks on its probable Cause. 359 



the retina itself. That luminous object is evidently the image 

 of the candle formed upon the retina, which becomes a ra- 

 diant point illuminating and rendering visible all the sur- 

 rounding parts of the eye, precisely as the image of the sun, 

 received through a small aperture into a darkened room, be- 

 comes not only a representation of the object, but a source of 

 illumination to everything in its vicinity. Upon this supposi- 

 tion the brown ground upon which the blood-vessels are seen 

 is no other than a picture of the retina itself, coloured, per- 

 haps, in consequence of its imperfect transparency, by a slight 

 reflection from the choroid coat beneath it. The singular 

 variations in the appearance of the foramen when the candle 

 is 18 inches distant from the eye are not at all at variance with 

 this solution. Illumination may be derived from the image 

 of the flame in two different ways, either in consequence of 

 the rapid curvature of the retina, by rays traversing a por- 

 tion of the vitreous humour in straight lines, or by rays gene- 

 rally diffused through the substance of the retina itself in the 

 vicinity of the image in consequence of its imperfect transpa- 

 rency. The latter mode of illumination will come into opera- 

 tion at those small distances where the former ceases to act 

 from the excessive obliquity of the ray to the illuminated sur- 

 face. The former produces the shadow observed at a mode- 

 rate distance from the flame; the latter, the luminous appear- 

 ance when almost in contact with it. The dusky spot in the 

 bottom of the foramen in this instance proves the thinness of 

 the retina in that place ; but its perfect uniformity of colour 

 with the surrounding parts in other cases seems to indicate 

 that it is not, as usually stated, deficient. This may very pro- 

 bably be a peculiarity of the eye of the observer, especially as 

 it is asserted in page 550 of the Report of the British Asso- 

 ciation, that to other obseners it appears as a dark spot. The 

 experiments seem to show that the foramen is encompassed at 

 its extreme edge with an opake fibre. Its total disappearance 

 at a considerable distance from the flame is a clear proof of 

 its shallowness. The difference of distinctness of the ramifi- 

 cations at different distances from the candle may probably 

 arise from the difference of the above-mentioned modes of 

 illumination. The imperfect transparency of the living retina 

 has been hitherto only inferred from its condition when de- 

 prived of life ; it may now be considered as established in a 

 very singular and unexpected manner both from the shadow 

 and the light which are observed under different circumstances 

 on the declivity of ihe foramen ccntralc. 



An observation of another kind may be here introduced as 

 having a remote bearing upon the same interesting subject. 



