1883.] 



Internal Reflexions in the Eye. 



475 



was at first incredulous, but being convinced when I showed him how 

 to find the ghost, he attributed it immediately to reflexion from the 

 fundus, and compared it with other retinal images. He wrote to me 

 after a day or two, saying he had found a complete solution of the 

 problem, and would like, if I had no objection, to publish an account 

 of some experiments he had made in connexion with the subject. 

 Later he told me he had given up the idea, and I have not seen or 

 heard of any published account. I had, however, made observations 

 which led me to question his theory, and I determined to make further 

 experiments, measurements, and calculation, to test my own view that 

 the reflexion was from the surface of the lens, and not from the fundus 

 alone. I was at one time inclined to consider that the ghost was con- 

 nected with " Sanson's images," but was driven from that idea by 

 considerations as to the amount of light that could be reflected back 

 into the eye ; this could only be extremely small. (See also below, 

 §30.) 



13. Many reasons hereinafter related make me believe that the 

 ghost is produced in the following way: — An image of the candle 

 flame is thrown on the retina by the crystalline lens; this image, 

 which I will call the first retinal image, may now be considered as a 

 source of light from which rays proceed outwards from the retina, 

 and are refracted by the crystalline lens and cornea to a focus again 

 outside the eye in such a way that an image of the retinal image 

 would be formed in the place occupied by the candle. Part of these 

 rays, however, are reflected at the various surfaces bounding the 

 different media of the eye, and those reflected by the anterior surface 

 of the lens, or what one should more correctly here regard as the 

 posterior surface of the aqueous humor, are brought to a focus some- 

 where between the lens and the retina. The rays from this focus 

 have not diverged much before they fall on the retina, and there form 

 a blurred inverted image of the first retinal image. This second 

 image is "referred" outwards, and looks as if produced by a faint 

 source of light outside the eye, and having a definite position in 

 space. 



14. I shall speak in what follows of this imagined source of light 

 as the "ghost," to distinguish it from the actual image of the retina 

 which gives rise to the sensation, and which I shall call the second 

 retinal image. The ghost is the analogue of the candle, each being 

 the mental image corresponding to the physical image on the retina. 

 The name ghost will suggest to the reader similar phenomena in the 

 telescope, which, however, are produced in a different way. 



15. The ghost is affected by accommodation, so that with the candle 

 about one foot from the eye, and with the eye accommodated for a 

 point six inches from it, the ghost does not become visible till it is 

 11 degrees from the centre of the field. These numbers refer to the 



