THE SENSES 905 



d its distance from B', we have = . Now, d may approxi- 

 mately be taken as 15 mm. Suppose, then, that the size of the 

 moon's image on the retina is required. Here 0238,000 miles, 

 and AB (the diameter of the moon) =2,160 miles. Thus we get 



238*000 =A I^' or (say) H^ = ^f"' from which A/B/ (the diameter 



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



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



120 feet 



throw on the retina an image whose height is - -, xi5 mm., 



25 miles D 



120 feet i 



i.e.,- - 7X15 mm., or- x 15 mm., equal to 0-013 mm., 



5,280 x 25 feet 1,100 



or 13 jw 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. 912) 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 cepha- 

 lopods, by altering the distance between lens and sensitive 

 surface ; in the eye of man, by altering the curvature, and there- 

 fore 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 accom- 

 modation 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 circum- 

 stances. 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 recognised : 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 



