ON THE MECHANISM OF THE EYE. 



589 



imagine, that the cornea might have some 

 slight effect in the natural state ; but, con- 

 sidering that the artificial cornea was about 

 -a tentli of an inch before the place of the 

 natural cornea, I calculated the effect of this 

 difference, and found it exactly sufficient to 

 account for the diminution of the range of 

 vision. I cannot ascertain the distance of 

 the glass lens from the cornea to the hun- 

 dredth of an inch ; but the error cannot be 

 much greater, and it may be on either side. 



After this, it is almost necessary to apo- 

 logize for having stated the former experi- 

 ments ; but, in so delicate a subject, we can- 

 not have too great a variety of concurring 

 evidence. 



VIII. Having satisfied myself, that the 

 cornea is not concerned in the accommoda- 

 tion of the eye, my next object was, to in- 

 quire if any alteration in the length of its 

 axis could be discovered ; for this appeared 

 to be the only possible alternative : and, 

 considering that such a change must 

 amount to one seventh of the diameter 

 of the eye, I flattered myself with the ex- 

 pectation of submitting it to measurement. 

 Now, if the axis of the eye were elongated 

 one seventh, its transverse diameter must 

 be diminished one fourteenth, and the semi- 

 diameter would be shortened a thirtieth of an 

 inch, 



I therefore placed two candles so that when 

 the eye was turned inwards, and directed to- 

 wards its own image in a glass, the light re- 

 flected from one of the candles by the scle- 

 rotica appeared upon its external margin, so 

 as to define it distinctly by a bright line : and 

 the image of the other candle was seen in 

 the centre of the cornea. I then applied the 

 tlouble eye glass, and the scale of the look- 



ing glass, in the manner already described j 

 but neither of them indicated any diminution 

 of the distance, when the focal length of the 

 eye was changed. 



Another test, and a much more delicate 

 one, was the application of the ring of a key 

 at the external angle, when the eye was 

 turned as much inwards as possible, and 

 confined at the same time by a strong oval 

 iron ring, pressed against it at the internal 

 angle. The key was forced in as far as the 

 sensibility of the integuments would admit, 

 and was wedged, by a moderate pressure, 

 between the eye and the bone. In this situ- 

 ation, the phantom, caused by the pressure, 

 extended within the field of perfect vision, 

 and was very accurately defined ; nor did it, 

 as I formerly imagined, by any means pre- 

 vent a distinct perception of the objects ac- 

 tually seen in that direction ; and a straight 

 line, coming within the field of this oval 

 phantom, appeared somewhat inflected to- 

 wards its centre; (Plate 9- Fig. 78.) a dis- 

 tortion easily understood by considering the 

 effect of the pressure on the form of the re- 

 tina. Supposing now the distance between 

 the key and tiic iron ring to have been, as it 

 really was, invariable, the elongation of the 

 e3'e must have been either totally or very 

 nearly prevented ; and, instead of an increase 

 of the length of the eye's axis, the oval spot, 

 caused by the pressure, would have spread 

 over a space at least ten times as large as the 

 most sensible part of the retina. But no such 

 circumstance took [dace . the power of ac- 

 commodation was as extensive as ever; and 

 there was no perceptible change, either in 

 the size or in the figure of the oval spot. 



Again, since the rays which pass through 

 the centre of the pupil, or rather through 



