Ofi the Mechamfm of the Eye. 29'9 



a diftortion eafily underftooil by confidering the effeft of the preffurt on the form of the 

 retina. Suppofing now, the diftance between the key and tlie iron ring to have been, as 

 it really was, invariable, the elongation of the eye muft have been either totally or very 

 nearly prevented ; and, inftcad of an increafe of the length of the eye's axis, the oval fpot 

 caufed by the preflure would have fpread over a fpace at leaft ten times as large as the moft 

 fenfible part of the retina. But no fuch circumftance took place : the power of accom- 

 modation was as extenfive as ever ; and there was no perceptible change, either in the fizc 

 or in the figure of the oval fpot. 



Again, fince the rays which pafs through the centre of the pupil, or rather the anterior 

 vertex of the lens, may, as already obferved, be confidered as delineating the image f and, 

 fince the divergence of thefe rays with refpe£l: to each other, is but little afFefted by the 

 refraflion of the lens, they may ftill be faid to diverge from the centre of the pupil ; and 

 the image of a given objedl on the retina muft be very confiderably enlarged, by the re- 

 moval of the retina to a greater diftance from the pupil and lens. (Cor. Prop. V*.) To 

 afcertain the real magnitude of the image with accuracy, is not fo eafy as it at firft 

 fight appears ; but, befides the experiment laft related, which might be employed as an 

 argument to this purpofe, there are two other methods of eftimating it. The firft is too 

 hazardous to be of much ufe ; but, with proper precautions, it may be attempted. I fix 

 my eye on a brafs circle placed in the rays of the fun, and, after fome time, remove it to 

 the cancellated micrometer ; then, changing the focus of my eye, while the micrometer 

 remains at a given diftance, I endeavour to difcover whether there is any difference in the 

 apparent magnitude of the fpedlrum on the fcale ; but I can difcern none. I have not in- 

 fifted on the attempt ; efpecially as I have not been able to make the fpeftrum diftin<St 

 enough without inconvenience ; and no light is fulTiciently flrong to caufe a permanent 

 impreffion on any part of the retina remote from the vifual axis. I therefore had recourfe 

 to another experiment. I placed two candles fo as exaftly to anfwer to the extent of the 

 termination of the optic nerve, and, marking accurately the point to which my eye was 

 direfted, I made the utmoft change in its focal length ; expefting that, if there were any 

 elongation of the axis, the external candle would appear to recede outwards upon the 

 vifible fpace. (Plate XIV. Fig. 15.) But this did not happen ; the apparent place of the 

 obfcure part was precifely the fame as before. I will not undertake to fay, that I could 

 have obferved a very minute difference either way : but I am perfuaded, that I fliould have 

 difcovered an alteration of lefs than a tenth part of the whole. 



It may be inquired if no change in the magnitude of the image is to be expe£led on any 

 other fuppofition ; and it will appear to be poffible, that the changes of curvature may be 



• This Corollary thould ftand thus. " If a confufed image be received on any given plane, it will be 

 neceffary, in order to determine its magnitude, to advert to the aperture admitting the rays. If tlie aperture 

 be fuppofed to be infinitely fmall, it may be confidered as a radiant point, in order to find the direftion of 

 tlie emergent rays." 



