264 On the Optical and Physiological Discoveries 



to act when the finger is applied. But such a supposition, if 

 at all intelligible, returns with equal force against the argu- 

 ment which it is brought to support, for the same disposition 

 to act, on the part of the muscles, ought to be conveyed to the 

 sensorium when the motion of the eyeball is produced by the 

 finger alone. 



I forbear saying any more on the subject, although the views 

 which I have given are susceptible of numerous illustrations. 



Those who are desirous of being acquainted with the funda- 

 mental difference between ordinary vision, and the vision of 

 impressions, will see it in the two figures on the board, the 

 lowest of which represents the effect produced, when the eyes 

 receive an angular motion of translation from the fingers, 

 while the upper one, which I gave formerly, shows the effect 

 produced when the eye is pushed so as to move parallel to it- 

 self. In the first case, the change of place of the spectrum, 

 which Mr Bell cannot see, is equal to the vertical ascent of the 

 eye, added to the tangent of its angular motion, while in the 

 second case, it is equal only to the vertical ascent of the eye ; 

 but in both cases, there is the same relative difference between 

 the change of place in ordinary vision, and the change of place 

 in spectral vision, a difference of such magnitude as to have 

 occasioned the oversight which Mr Bell has committed.' 1 



Mr Bell's paper on the Motions of the Eye-ball, which was 

 read after the above notice, was a controversial reply to a 

 paper inserted in Number iii. of this Journal, and was print- 

 ed in a periodical work now at an end. It was read by his 

 brother, George Joseph Bell, Esq. advocate, who supported 

 the contents of his scientific brief by the system of attack and 

 defence which is still tolerated at the bar. There were present 

 at the meeting of the Royal Society, a very great number of 

 medical gentlemen, and among them several of the most dis- 

 tinguished medical professors in the University ; and as soon 

 as Mr Bell's paper was read, Dr Brewster rose, and pointed 

 out to the society the various errors which Mr Bell had com- 

 mitted, both in his experiments and in his reasoning, and he 

 pledged himself to answer any objection, or explain any diffi- 

 culty, which any of the gentlemen present might state ; and 



