2t2 Viin.itUiis cf Curvature ill the Conica. 



diirorcnt didanccs, does not rclidc in the cryflalliiie lens ; at lead not altogether ; and that it 

 any agency in tliis rcfpeiSl can be proved to rcfide in the cryftalllne, the other powers, what- 

 ever they may be, are capable ol exertion beyond their ufual limits, fo as to perform its 

 office in this refpe£l. 



I'roni liicfe confiderations; and in confequencc of olher reflexions tending to fliew that 

 an elongation of the optical axis is not probably the means of adjuftment, thofe philofophers 

 direiled their enquiries to afcertain how far the curvature of the cornea might be fubjcifl to 

 change. They found by trial that this part of the organ polVcfles a degree of el.ifticity which 

 is very confideriible, both for its perfe£lion and its range ; and by anatomical dilVciSlion it 

 was found, that the four llraight niufcles of the eye do in eflecl terminate in the cornea at 

 their tendinous extremities ; that the whole external lamina of the cornea could by gentle 

 force be feparatcd, by means of thefe mufcles, from the eye ; fo that the tendons feem loft 

 in the cornea, and this lallhas the appearance of a central tendon. It was alfo feen that the 

 central part of tlic cornea is thethickcft and the mod claftic. 



Thefe were confidcrable advances towards eftablifhing the hypothefis of adjuftment by 

 the external curve of the eye. It remained to be (hewn, by experiments on the living fub- 

 jeiSV, that this curve does really vary in the due dire£lion, when the mind perceives the dif- 

 tiniSl vifible fenfation of objefts at diflcrcnt diflances. For this purpofe Mr. Ramfden pro- 

 vided an apparatus, confiftlng of a thick board fleadily fixed, in which was a fquare hole large 

 enough to admit a pcrfon's face ; the forehead and chin reding againd the upper and lower 

 bars, and the cheek againd either of the fides ; fo that when tlie face was protruded, the 

 head was deadily fixed by reding on three fides ; and in tliis pofition the left eye projcfted 

 beyond the outer furfaceof the board. A microfcope, properly mounted, fo as with eafe to 

 be fetin every requifite pofition, was applied to view the cornea with a magnifying power of 

 thirty times. In this fituation, the pcrfon whofe eye was the obje£l of experiment was 

 dcfired to look at the corner of a chimney, at the didance of 235 yards, through a fniall 

 hole in a brafs plate, fixed for that purpofe, and afterwards to look at the edge of the hole 

 itfclf, which was only fix inches didant. After fome management and caution, which the 

 delicate nature of thefe experiments requires, the motion of the cornea, which was imme- 

 diately perceptible, became very didinifl and certain. The circular feftion of its furface 

 remained in a line with the wire in the field of the microfcope, when the eye was adjufted 

 to the didant object, but projected confiderably beyond it when adapted to the near one. 

 When th^ didant obje£l was only 90 feet from the obferver, and the near objeft fix inches, 

 the difference in the prominence of the cornea was edimated at i -Sooth of an inch. Thefe 

 experiments were repeated and varied at different times and on different fubjeds. The 

 obferver at the microfcope found no diflieuhy in determining, from the appearance of the 

 cornea, whether the eye was fixed on the remote or the near obje£t. 



From thefe diflercnt experiments Mr. Home confiders the following fa£ls to have been 

 afcertained : 



I. Tiiat the eye has a power of adjuding itfelf to different diftances when deprived of the 

 crydalline lens ; and therefore the fibrous and laminated drudure of that lens is not in- 

 tended to alter its form, but to prevent rededlions in the pafl'igc of the rays through the 

 furfaccs of media of different dcnfities, and to correct fpherical aberration. 

 ;,^. That the cornea is made up of laminx ; that it is claftic, and when ftrctclied is capable 



of 



