57S 



ON THE MECHANISM OF THE EYE. 



Ihble III. For concave slasses. 



Focus. 

 2.7s 



2. 50 



a. 2 5 



2.00 

 1.75 

 1.50 



V. Being convinced of the advantage of 

 making every observation with as little as- 

 sistance as possible, I have endeavoured to 

 confine most of my experiments to my own 

 eyes; and Ishall> in general, ground my calcu- 

 lations on the supposition of an eye nearlj' 

 similar to my 'own. I shall therefore first 

 endeavour to ascertain all its dimensions, and 

 all its faculties. 



For measuring the diameters, I fix a small 

 key on each point of a pair of compasses; 

 and I can venture to bring the rings into im- 

 mediate contact with the sclerotica. The 

 transverse diameter is externally 98 hun- 

 dredths of an inch. 



To find the axis, I turn the eye as much in- 

 wards as possible, and press one of the keys 

 close to the sclerotica, at the external angle, 

 till it arrives at the spot where the spectrum 

 formed by its pressure coincides with the di- 

 rection of the visual axis, and, looking in a 

 glass, I bring the otl^er key to the cornea. 

 The optica] axis of the eye, making allow- 

 »nce of three hundredths for the coats, is 

 thus found to be 91 hundredths of an inch, 

 from the external surface of the cornea to 

 the retina. With an eye less prominent, this 

 method might not have succeeded. 



The vertical diameter, or rather chord, of 

 the cornea, is 45 hundredths: its versed sine, 

 ] ] hundredths. To ascertain the versed sine, 

 I ;:)oked with the right eye at the image of 



the left, in a small speculum held close to the 

 nose, wiiile the left eye was so averted, that 

 the margin of tiie cornea appeared as a 

 straight line, and I then compared the pro- 

 jection of ilic cornea with the image of a 

 cancellated scale held in a pro[,er direction 

 behind the left eye, and close to the left 

 temple. The horizontal chord of the cor- 

 nea is nearly 49 hundredths. 



Hence the radius of the cornea is 31 hun- 

 dredths. It may be thought, that I assign 

 too great a convexity to the cornea ; but I 

 have verified it by a number of concurrent 

 observations, which will be enumerated here- 

 after. 



The eye being directed towards its image, 

 the projection of the margin of the sclerotica 

 is 22 hundredths from the margin of the 

 cornea, towards the external angle, and 27 

 towards the internal angle of the eye : so that 

 the cornea has an eccentricity of one for- 

 tieth of an inch, with respect to the section 

 of the eye perpendicular to the visual axis. 



The aperture of the pupil varies from 27 to 

 13 hundredths ; at least this is its apparent 

 size, which must be somewhat diminished, 

 on account of the magnifying power of the 

 cornea, perhaps to 25 and 12. When di- 

 lated, it is nearly as eccentric as the cornea ; 

 but, when most contracted, its centre coin- 

 cides with the reflection of an image from an 

 object held immediately before the eye; and 

 this image very nearly with the centre of the 

 whole apparent margin of the sclerotica : so 

 that the cornea is perpendicularly intersected 

 by the visual axis. 



My eye, in a state of relaxation, collects, 

 to a focus on the retina, those rays which 

 diverge vertically from an object at the dis- 

 tance of ten inches from the cornea, and the 

 rays which diverge horizontally from an ob- 



