1883.] 



Internal Reflexions in the Eye. 



479 



with, the reflexion sometimes to be seen on the scale of a reflecting 

 galvanometer. The light reflected from the mirror is partially re- 

 flected at the surfaces of the worked glass which closes the mirror 

 tube : and that part reflected within the glass at the surface remote 

 from the mirror is again reflected within the glass at the surface next 

 the mirror, so that a second and very faint image of the slit is some- 

 times formed on the scale. This second image moves in the same 

 direction as the ordinary bright one, but with greater angular 

 velocity. 



31. That the ghost is not due to reflexions within the lens is also 

 made very probable from calculations which show that in any state of 

 accommodation the focus would be virtual and situated in the aqueous 

 humor. That it is not due to reflexions within the aqueous humor is 

 made more probable from the fact that I have discovered a second 

 ghost, which is almost certainly due to this causey it moves in the 

 same direction as the candle, but the faintness and indistinctness of 

 this image is too great to allow of any accurate measurements. It is 

 very blurred, and I have never been able to bring it even approxi- 

 mately to a focus on the retina. 



32. The wish, to see the ghost, if possible, in the line of direct 

 vision has led me to further very interesting observations. The 

 method was suggested by an accidental sight of the ghost in bright 

 sunlight. I was standing on the terrace at Heidelberg, and looking 

 towards the sunset ; in front of me on the other side of the ravine 

 was the Castle, standing out dark against the bright sky beyond. As 

 my eye wandered slowly over the outline of the buildings, my atten- 

 tion was caught by the ghost flitting down below in the shadow. 

 Here were all the necessary conditions, a bright light and a dark 

 screen. There was no difficulty in moving the eye till the ghost fell 

 exactly on the point of direct vision. But, strange to say, the sun 

 was still some way out of the line of direct vision ; that is, instead of 

 seeing the sun and its ghost equidistant from the centre of the field, 

 and finally, as the eye moved further, merging into one at the centre 

 of the field, the ghost had got to the centre first, and had moved on 

 to the same side of the centre as the sun before they coincided. 



33. Moreover the eccentricity, as I may call it, is different for my 

 two eyes both in amount and direction. I can best describe the 

 difference by saying that with my right eye r when the ghost was in 

 the line of clearest vision, the sun was still to the right and above ; 

 and when the line through my eye was horizontal, the line through 

 the sun's position and the ghost, or the point of clearest vision, was 

 inclined at an angle of 36 degrees to the horizon, the angular distance 

 of the sun from the ghost being 8° 32'. With my left eye, when the 

 ghost was in the line of clearest vision, the sun was to the left on the 

 same level, and at an angular distance from the ghost of 6° 25'. 



