476 



Mr. H. F. Newall. 



[Jan. 25, 



case of my own eyes, and will, of course, vary with the observer. I 

 may add here that I am what would be called long-sighted, though 

 having a very large range of accommodation. 



16. Again, when the eye is focussed for a distant point, the ghost 

 is visible right up to the middle of the field, but invisible when 

 removed more than 17 degrees from the centre of the field. 



17. Again, if the eye- and candle are fixed whilst the eye is focussed 

 for different distances, the ghost seems also to pass through regular 

 gradations of distinctness. (See also below, § 35.) 



18. Now, in the last ease, it is clear that the candle can only give a 

 clear image on the retina in one particular state of accommodation, so 

 that generally speaking there will be a blurred first retinal image, the 

 reflexion of which is the second image. One would then expect that 

 the ghost would be more likely to be distinct when the candle is at 

 the same distance from the eye* as the point focussed. 



19. This I have found from observation to be the case when the 

 eye is focussed for points not nearer than two- feet. When the eye is 

 focussed for a point six inches from it, the ghost is invisible when the 

 candle is further than one foot from the eye r and in no position of the 

 candle does it become very distinct, but is most so when the candle is 

 close to the eye. With so close a point of regard the radius of cur- 

 vature of the anterior surface of the lens would be much diminished, 

 and hence also its focal length when regarded as a concave mirror, so 

 that the rays proceeding from the focus have diverged considerably 

 more before reaching the retina to give rise to the second image than 

 in the case when the eye is focussed for long- distances. 



20. Here it may be noticed that with the point of regard at arm's 

 length, the ghost increases much in size if the candle is approached to 

 the eye. And also if, with the eye focussed for a near poiut and the 

 candle about a foot from it r the point of regard be moved gradually 

 away from the candle across the field, then the ghost, which is not to 

 be seen in tho centre of the field, comes into sight and grows more 

 distinct as it becomes more moved from the centre up to a certain 

 point, beyond which it again becomes indistinct and finally disappears. 

 The point of distinctness varies also with the state of accommoda- 

 tion. 



21. From considerations referred to in § 18, the question arises : 

 Are we to' regard the retina in this matter as a concave mirror, or 

 merely as- a screen ? An example will help to make the importance of 

 this clear. Suppose the source of light is the sun ; then if the eye is 

 accommodated to see it distinctly, the focus of rays from the sun 

 would fall on the retina, which in this case would act simply as a 

 screen, receiving a distinct image of the sun, whence rays would 

 proceed outwards again as in § 13. If, however, the eye was accom- 

 modated for near distances, then rays from the sun would be brought 



