Vision of the Blood-vessels of the Eye. 119 



die : and I have besides ascertained that it has no focus ; for if 

 it had, it would expand and contract by any variation in the 

 distance of the candle. It cannot proceed from any imperfect 

 transparency in any of the coats or humours of the eye, be- 

 cause it is seen in eyes that have the most pure and perfect 

 vision. It must, therefore, be light produced by a physiolo- 

 gical action, or light propagated from, or induced by, the di- 

 rect image of the candle upon the retina; and if this is the 

 case, the explanation which I formerly gave of the phenome- 

 non is likely to be the true one. The blood-vessels touching 

 the retina will deaden, as it were, the part of the retina which 

 they touch, or make it less sensible to the propagated light, and 

 hence the blood-vessels will appear delineated in a fainter light 

 than thatwhich surrounds them. The distinctness with which 

 the ramifications will thus be seen will vary with the intensity 

 of the brown light, with the ever changing sensibility of the 

 retina, and with the varying pressure of the blood-vessels 

 themselves. If I could command the vision of these ramifica- 

 tions as Mr. Wheatstone can, it would not be difficult to in- 

 stitute experiments by which the preceding explanation could 

 be cross-examined; and I therefore hope that he will resume 

 the subject in reference to the facts and views which I have 

 ventured to state. 



Before concluding this notice I may mention, as connected 

 with the subject, some curious phasnomena which appear 

 when we throw a condensed beam of light upon the retina so 

 as to fill the whole eye. This may be done by holding near 

 the eye a convex lens, about an inch in diameter, and an inch 

 or so in focal length, so as to see its whole area filled with the 

 light of a candle or lamp. If we move the lens backwards 

 and forwards quickly, looking steadily at one point of the 

 field, we shall see on each side of the axis of vision the rami- 

 fications described in my former paper and in the preceding 

 pages. There are none of them visible within a certain space 

 round the axis of vision ; but in the axis of vision there is an 

 irregularly illuminated or shaded circular spot, obviously cor- 

 responding to the forameyx centrale of the retina ; and in this 

 spot, and for some distance round it, is seen a sort of net- 

 work pattern, delineated in dark lines. This pattern* has 

 sometimes all the regularity of one formed geometrically, with 

 dark spots in the centre of each area, and the ground on which 

 the pattern is seen is generally of a faint purple colour. But, 

 what is more remarkable, the luminous field is crossed by ex- 



• The very same phaenomenon is seen, though less distinctly, when we 

 look steadily at the moving or flaring summit of the flame of a candle. 



