786 



A MANUAL OF PHYSIOLOGY 



_^l6o_ = Air ( )_I_ ; 

 238,000 15 



A'B' 



, from which A'B' (the diameter 



no 15 



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



I TO 



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

 throw on the retina an image whose height is 20 x 15 mm., 



i.e., 



120 feet 



x 15 mm., or 



25 miles 

 x 15 mm., equal 10-013 mm., 



5,280 x 25 feet 1,100 



or 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. 794) 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 alter- 

 ing the relative position of 

 the lenses ; in a photo- 



FIG. 292. PURKINJE-SANSON IMAGES. 



A, in the absence of accommodation ; B, 

 during accommodation for a near object. 

 The upper pair of circles enclose the images 



as seen when the light falls on the eye graphic Camera and in the 



through a double slit or a pair of prisms ; - ,, , , , 



the lower pair show the images seen when eyes Ol fiSties and Cepna- 



triangular in shape. lopods (Beer)> by altering 



the distance between lens and sensitive surface ; in the 



