On tBe Mechanifm of the Eye. 327 



agreeable,' that I thought for a time that this property was communicated by the vapours of 

 the bituminous combuftlble which is ufed in this country, and I did not give up-my opi- 

 nion till I had found myfelf as ftrongly afFefted with it in the open country as in the town* 

 My eyes were irritated fo as to flow with involuntary tears. I returned home after expe- 

 riencing the greatefl: difficulties, and obferving that the fog in the ftreets was much more 

 elevated than in the open country. I obferved, as Citizen Fourcroy did on a former oeca- 

 fion at Paris, that it was more eafy to find the way in the narrow ftreets, than in fuch as 

 were wider, in the former of which the fog defcended with much lefs rapidity. 



in. 



On the Mechanifm of the Eye. By Thomas Yovng, M.D. F.R. S. 

 f Continued from page 303.^ 



H. 



. ITHERTO I have endeavoured to fhew the inconveniences attending other fuppofitions j 

 and to remove the objections to the opinion of an internal change of the figure of the lens. 

 I (hall now ftate two experiments, which, in the firft place, come very near to a mathema- 

 tical demonftration of the exiftence of fuch a change, and, in the fecond, explain in great 

 meafure its origin, and the manner in which it is efFefted. 



I have already defcribed the appearance of the imperfeft image of a minute point at different" 

 dlftances from the eye, in a ftate of relaxation. For the prefent purpofe, I will only re- 

 peat, that if the point is beyond the furtheft focal diftance of the eye, it aflumes that: 

 appearance which is generally defcribed by the name of a ftar, the central part being con- 

 fiderably the brighteft. (Plate XV. Fig. 36— 39.) But, when the focal diftance of the eye- 

 is Ihortened, the imperfe£t image is of courfc enlarged ; and, befides the neceflary confe- 

 quence, the light is alfo very diiFerently diftributed ; the central part beomes faint, and the 

 margin ftrongly illuminated, fo as to have almoft the appearance of an oval ring. (Fig. 41.) 

 If I apply the Aider of the optometer, the ftiadows of the flits, while the eye is relaxed, are 

 perfedtly ftraight, dividing the oral either way into parallel fegments : (Fig. 42, 44.) but,, 

 when the accomodation takes place, they Immediately become curved, and the more fo the 

 further they are from the centre of the -image, to which their concavity is dire£ted. 

 (Fig. 43, 45.) If the point be brought much within the focal diftance, the change of the 

 eye will increafe the illumination of the centre, at the expenfe of the margin. The fame 

 appearances are equally obfervable, when the efFe£l of the cornea is removed by immer- 

 fion in water i and the only imaginable way of accounting for the diverfity, is to fuppofe 

 the central parts of the lens to acquire a greater degree of curvature than the marginal parts. 

 If the refra£lion of the lens remained the fame, it is abfolutely impoCTible that any change 

 of the diftance of the retina Ihould produce a curvature in thofe fliadows, which, in the 

 3 relaxedv 



