Dr. Brewster on the Vision of Impressions on the Retina. 5 



ost, the eye must either see the object erect as usual, or in- 

 verted, or in some intermediate position, or what is more pro- 

 bable, in all these positions at once. For if it has a determi- 

 nate position, the eye will only have exchanged its notion of 

 true position for a notion of false position, a result too absurd 

 to be for a moment entertained. Fortunately for this argu- 

 ment, Mr. Bell has actually described a case under the care 

 of Dr. Macmichael, which occurred after his paper was read . 

 « In this case," says he, « which shows the consequences of 

 the eye and eyelids being rendered immoveable, the surface 

 of the eye is totally insensible, and the eye remains fixed and 

 directed' straight forward, whilst the vision is entire." If 

 there ever was an experimentum cruets, which could settle at 

 once a controverted question, we have one in the case now 

 quoted. Dr. Macmichael's patient preserved his vision en- 

 tire, when " the outward apparatus was without sensibility 

 and' motion," and when there was no consciousness of effort 

 in the voluntary muscles to convey the notions of place and 



relation. 



Although mathematicians have acknowledged the legitima- 

 cy of the reductio ad absurdum, which constitutes the princi- 

 pal feature of the preceding argument, yet we fear this will 

 not be admitted in physical science, unless it is accompanied 

 with an acknowledgment of our ignorance respecting the facts 

 and principles which the paralogism involves. I shall there- 

 fore proceed to an examination of the facts themselves. 



1. The leading fact which has misled Mr. Bell in this in- 

 quiry, is the alleged immobility of the spectral impression, 

 when the eye is displaced by the pressure of the finger. 

 This spectrum is by no means immoveable. It is quite true 

 that it moves through a very small space ; but this space, 

 small as it is, is the precise quantity through which it ought 

 to move according to the principles of optics ; and the expla- 

 nation of this fact leads us to investigate the difference be- 

 tween the vision of external objects, and that of impressions 

 upon the retina. 



In order to understand this difference, let A, Plate II. Fig. 

 1, be the eye of the observer, and O an external object, 

 whose image at P is seen along the axis of vision POM. 

 Let the eye be pushed upwards, suppose ^th of an inch, 



