478 



Mr. H. F. Newall. 



[Jan. 25, 



and more of the figure of the ghost : but I find, on the contrary, that 

 it dies away gradually, getting " out of focus," as it were, as it 

 passes from the field. 



27. On the other hand, the anterior surface of the lens seems most 

 likely to give the solution of the problem. Calculation shows that 

 the light proceeding outwards from the first image on the retina 

 would be, after refraction, at the posterior surface of the lens, 

 reflexion at the anterior surface, and second refraction at the posterior 

 surface, focussed at a point in the vitreous humor. This point, when 

 the eye is focussed for long distances, will be about 2 "4 millims. from 

 the posterior surface of the lens : and when the eye is focussed for a 

 near point, it will be about 1 millim. from that surface. The distance 

 of these points from the retina certainly forms a very great difficulty 

 in the explanation. 



28. As to the brightness or rather faintness of the image, it seems, 

 at first sight, improbable that the surface between the lens and 

 aqueous humor should be capable of reflecting enough of the light 

 coming from the first retinal image, to excite the retina. But the 

 case is comparable with the appearance of " Sanson's Images," and 

 the brightness of the ghost, as compared with the candle, might well 

 be described as about the same as the two faint images as compared 

 with the bright one in Sanson's phenomenon. "We might expect to 

 find some change in the brightness of the ghost when the first retinal 

 image falls on the more opaque part of the retina at the entry of the 

 optic nerve. But I have failed to get certain results. I have allowed 

 the image of the candle to fall on the blind spot : the ghost does not 

 lose in brightness, though the whole field of vision seemed to become 

 brighter, in consequence, no doubt, of the greater diffusion of light 

 through the eyeball from the image on that part. That the ghost 

 does not seem to lose in brightness as the ground becomes more 

 illuminated, may, perhaps, be taken as a sign of increased brightness ; 

 but nothing definite can be gained from this point towards the 

 explanation. 



29. The fact that the ghost is visible in the centre of the field (see 

 below, § 32) shows that the reflexion cannot be from the fundus 

 alone, that is from the fundus direct, on to another part of the retina, 

 as I understood Mr. Liebreich to have imagined. This is clear, more- 

 over, from the fact that if the fundus were the only reflector in the 

 case, the ghost would move in the same direction as the candle. This 

 would also be the case if the first reflexion outwards were from a 

 surface in front of the retina instead of the retina itself. 



30. If, for instance, the reflexions were entirely within the lens (as 

 I at first thought possible) or within the aqueous humor, then the 

 ghost and candle ought to move in the same direction, the former 

 with greater angular velocity. In fact, the case would be comparable 



