980 THE SENSES 



Thus we get -^ l6o _=^L?-', or (say) -L = ^?', from which A'B' (the 

 238,000 15 no 15 



diameter of the retinal image) = ^-, or about \ mm. 



A ship's mast 120 feet high, seen at a distance of 25 miles, will throw 



120 feet 

 on the retina an image whose height is x 15 mm., i.e., 



120 feet i 



7 7x15 mm., or x 15 mm., equal to 0-013 mm., or 



5,280 x 25 feet 1,100 



13 p in size. This is not much larger than a red blood-corpuscle, and 

 only four times the diameter of a cone in the fovea centralis, where the 

 cones are most slender. In this calculation the effect of aberration 

 (p. 987) in enlarging the image has been neglected. This effect is, of 

 course, proportionately greater for small and distant than for large and 

 near objects; and it is doubtful whether the smallest possible image 

 can be confined to an area of the retina of the size of a single cone. 



Accommodation. A lens adjusted to focus upon a screen the 

 rays coming from a luminous point at a given distance will not be 

 in the proper position for focussing rays from a point which is 

 nearer or more remote. Now, it is evident that a normal eye 

 possesses a great range of vision. The image of a mountain at a 

 distance of 30 miles, and of a printed page at a distance of 30 cm., 

 can be focussed with equal sharpness upon the retina. In an 

 opera-glass or a telescope accommodation is brought about by 

 altering the relative position of the lenses ; in a photographic camera 

 and in the eyes of fishes and cephalopods, by altering the distance 

 between lens and sensitive surface ; in the eye of man, by altering 

 the curvature, and therefore the refractive power of the lens. That 

 the cornea is not alone concerned in accommodation, as was at one 

 time widely held, is shown by the fact that under water the power 

 of accommodation is not wholly lost. Now, the refractive index 

 of the cornea being practically the same as that of water, no changes 

 of curvature in it could affect refraction under these circumstances. 

 That the sole effective change is in the lens can be most easily 

 and decisively shown by studying the behaviour of the mirror 

 images of a luminous object reflected from the bounding surfaces 

 of the various refractive media when the degree of accommodation 

 of the eye is altered. Three images are clearly recognized: the 

 brightest an erect virtual image, from the anterior (convex) surface 

 of the cornea; an erect virtual image, larger, but less bright, from 

 the anterior (convex) surface of the lens; and a small inverted real 

 image from the (concave) posterior boundary of the lens (Purkinje- 

 Sanson images). The second image is intermediate in position 

 between the other two. It is possible with special care to make 

 out a fourth image; but since it is reflected from the posterior 

 surface of the cornea, at which only a slight change in the refractive 

 index occurs, it is less brilliant than the first three. When the eye 

 is accommodated for near vision, as in focussing the ivory point of 



