1825.] mmfo/vMon. 259 



facts in support of the base of the optic nefve being the chief 

 agent in our particular sensations,— still leaving the correct 

 solution of these matters, however, open to further inquiry. 



In every'&ct of vision it may be proved by observation, that 

 whether we employ one only, or both, of our eyes in conjunc- 

 tion^ wbjle looking towards any object, there is -a particular , as 

 vi'eir M '^ 'general sensation, both of which are occasioned by 

 tb^'coiistitution of our organs of vision. The particular sensa- 

 tibh is this, that the eye discovers distinctlij but a single point 

 only in the act of employing it ; this point being comparatively 

 small or great, according as the object contemplated is near 

 the eye, or remotely distant from it. The general sensation is 

 thi^, that the eye indistinctly discovers a very considerable 

 space, v^hich is greater or less according to the pecuharities of 

 its constitution, or the quantity of light to which it is exposed ; 

 that is, according to the convexity of the cornea, and the dila- 

 tation and contraction of the pupil. The particuleir sensation, 

 it vi^ill be obvious, is here' referred to the base of the optic 

 nerVe, and the general sensation to the retina, resting upon the 

 intfernal coat of the eye. ; 



"Here, however, it will be n^ee^^4ty tt>' state other reasons fof 

 these conclusions, than those reflections from the interior of the 

 eye upon the lens already alluded to in observing the moon, 

 which, notwithstanding, are of themselves sufficiently obvious; 

 but it may, first of all, be proper to show in what manner it can 

 be provedi that there is a particular, as well as a general sensa- 

 tiOn,' — a circumstance, although extremely simple in itself, yet, 

 never before mentioned, that I am aware of, by any writer. 

 This may be done by making a minute centre, and describing a 

 few circles round it, with radii of any number of parts of an 

 itifetfi ^ Wi*h- W^small test of this kind it will be found that at 

 the* disM^fe^ of between three and six inches, according to cir^ 

 cumstances, the central point only of this diagram can be 

 distinctly seen at the same instant, and that the eye may be 

 directed round the smallest of the circles without receiving a 

 particular sensation of the centre. The same thing may be 

 observed, however, in reading, when it will be found that we 

 c^ti distinctly trace but one letter at a time, and scarcely thi&^ 

 and that it is only by directing the eye upon each letter in sHca 

 cession that we are enabled to comprehend the sense, except 

 so far as we are otherwise assisted by- m?einory, in ascertaining 

 the Whole fi-om a part. ^^ -ioo^nhdu ■.] 



'I now proceed to offer a few remarks respecting the probable 

 cause of a particular and a general sensation arising on directing 

 the eye towards any object; for which purpose I shall first 

 allude to a circumstance towards which my attention has been 

 frequently directed, and afterwards towards one which; was 

 taken notice of in the last century by a number of philosophersy 



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