VOL. LXXXV.] PHILOSOPHICAL TKANSACTIONS. 455 



surfaces of the lenses, which by diluting the image formed in the focus of the 

 object glass, makes that image appear far less bright than it would otherwise have 

 done, producing that milky appearance so often complained of in viewing lucid 

 objects through this sort of telescope. In the eye, the same properties that ob- 

 viate this defect, serve also to correct the errors from the spherical figure, by a 

 regular diminution of density from the centre of the crystalline outward. Every 

 appearance shows the crystalline to consist of laminae of different densities ; and 

 if we examine the junction of different media, having a very small difference of 

 refraction, we shall find that we may have a sensible refraction without reflection : 

 now if the difference between the contiguous media in the eye, or the laminae in 

 the crystalline, be very small, we shall have refraction without having reflection, 

 and this appears to be the state of the eye ; for though we have 2 surfaces of the 

 aqueous, 2 of the crystalline, and 2 of the vitreous humour, yet we have only 1 

 reflected image, and that being from the anterior surface of the cornea, there 

 can be no surface to reflect it back, and dilute an image on the retina. 



This hypothesis may be put to the test, whenever accident shall furnish us 

 with a subject having the crystalline extracted from one eye, the other remaining 

 perfect in its natural state; at the same time we may ascertain whether the crystal- 

 line be that part of the organ which serves for viewing objects at different dis- 

 tances distinctly. Seeing no reflection at the surface of the crystalline might 

 lead some persons to infer that its refractive power is very inconsiderable, but 

 many circumstances show the contrary ; yet what it really is may be readily 

 ascertained, by having the focal length and distance of a lens from the operated 

 eye, that enables it to see objects the most distinctly ; also the focal length of a 

 lens, and its distance from the perfect eye that enables it to see objects at the 

 same distance as the imperfect eye ; these data will be sufficient for calculating 

 the refractive power of the crystalline with considerable precision. Again, having 

 the spherical aberration of the different humours of the eye, and having ascertained 

 the refractive power of the crystalline, we have data from which to determine the 

 proportional increase of its density as it approaches the central part, on a sup- 

 position that this property corrects the aberration. 



These observations of Mr. Ramsden respecting the use of the crystalline lens, 

 I was very desirous of bringing to the proof; and while my mind was strongly 

 impressed by them, a favourable opportunity occurred. A young man came into 

 St. George's hospital with a cataract in the right eye: this proved to be a fair case 

 for an operation, to which the man very cheerfully submitted, and was put under 

 my care for that purpose. In performing the operation, the crystalline lens was 

 very readily extracted, and the union of the wound in the cornea took place un- 

 attended by inflammation, so that the eye suffered the smallest degree of injury 



