VISION 393 



Cylindrical glasses, not lenses, are required to correct this 

 defect. And here it may be well to call attention to the fact 

 that rays of light are more sharply refracted by the surface of 

 the cornea than they are by the crystalline lens. The lens has 

 a high index of refraction (1-45), but it does not lie in air (the 

 index of refraction of which is 1), but between two humours 

 which have about the same index as water namely, 1-336. The 

 bending by the combined action of the cornea and the lens of 

 rays of light which come from a source so distant that they may 

 be considered as parallel brings them to a focus on the retina, 

 when the lens is at its flattest. When the lens is at its roundest, 

 rays which diverge from a point only 5 inches in front of the 

 eye are focussed on the retina. The lens is therefore essential 

 for accommodation, but, after its removal for cataract, vision, 

 even for near objects, is rendered possible by the use of convex 

 glasses. 



A star or a distant gas-lamp is seen as a point of light with 

 rays. Usually this figure, which has given origin to the 

 expression " star-shaped," shows three greater rays alternating 

 with three lesser rays. Such an image is not produced by a 

 point of light near to the eye, since it is due to the puckering 

 of the lens when flattened against its ligament. It brings 

 into evidence the three axes on the front of the lens and the 

 three axes which alternate with them on the back, with regard 

 to which the lens-fibres are disposed. 



As an adaptation of living tissues to optical purposes the 

 eye is above admiration, yet it presents many defects, which 

 an optician corrects in the instruments which he manufactures. 

 A remarkable fact in the physiology of vision is our uncon- 

 sciousness of the imperfections of its organ. An unusual 

 experiment is needed to bring them to our notice. If we look 

 through a common glass lens uncorrected for unequal refrac- 

 tion of rays of different wave-lengths, we recognize that a 

 bright object is shown with a colour-fringe, yet we take no 

 cognizance of the colour-fringes which surround the images of 

 all bright objects focussed upon our retinae. If we think about 

 the matter, we recognize a feeling that blue in a window of 

 stained glass appears farther away than red ; but this might 

 well be due to association. Blue glass is chiefly used for the 

 sky. If we look at a bright object through purple glass, we 



