Mr. Rumball on the Focus of Vision. 311 



I shall reserve for a future occasion some remarks which 

 I have to make upon the errors, apparent in the diagrams of 

 all the books 1 have consulted upon this subject, and confine 

 myself at present to my own proposition and its proofs. 



I affirm that " the images of objects are not inverted upon 

 the retina, but that every image is painted there in a point ;" 

 in other words, ^'^ the focus of the eye is upon, and not anterior 

 to, the retina" 



I herewith send you two experiments, which I purpose to 

 follow up by some others, equally conclusive; but which 

 would occupy too much space if inserted in this paper. 



Exp. 1. — Dissect off the coats of the eye at the posterior ex- 

 tremity of its axis. Hold it up between the finger and thumb, 

 and the vitreous humour will protrude as in fig. 1. A, the 



Fiff. 1. Fig. 2. 



pupil. B. the axis of the eye. C. the protruding vitreous 

 humour. Look through the eye, and pass a probe or other 

 object backwards and forwards before the pupil. Its ap- 

 parent motion will be the reverse of its real one. 



Exp. 2. — Cut away the protruding vitreous humour, and 

 the eye will assume the shape of fig. 2. A. the pupil. B. the 

 axis of the eye. C. the depression occasioned by the abstrac- 

 tion of the humour. 



Now pass a probe up and down before the pupil ; and upon 

 looking through the eye, the probe's real and apparent motion 

 will be the same. 



In the first experiment the axis of the eye is slightly elon- 

 gated; and as the image of the probe is inverted, we have suf- 

 ficient proof that the rays of light proceeding froin it must 

 have crossed each other, before their exit irom the eye. The 

 focus is there/ore within the axis. 



In the second experiment, the axis is shortened and the 

 real and apparent position of the object being the same, demon- 

 strates that the rays have not crossed ; consequently the focus 

 of the eye is now without, or posterior to it. But as the retina 

 is situate between tiie point where the first Experiment proves 



New Series. Vol. 2. No. 11. Nov. 1827. 3 C the 



