244 LII'E : OUTLINES OF GENERAL BIOLOGY 



air. Moreover, there is a special fascination for the physicist in the 

 properties of the lens; it has greater refractive power than its 

 composition and shape would suggest, because at the centre it is 

 not merely thicker but also denser than towards the circumference; 

 and, finally, it has in birds and mammals the power of changing 

 its shape for the purpose of focusing. In fishes, amphibians, and 

 some reptiles, as in some invertebrates, the eye focuses as a camera 

 docs — the distance between the lens and the retina is altered, by 

 one means or another; but in other reptiles, in birds and in mammals, 

 a new principle has been discovered. A lens, removed from the eye, 

 has the shape of a much-flattened sphere; but in position it is com- 

 pressed by the ligaments which hold it, is still more flattened, and 

 is then able to throw sharp images of distant objects upon the 

 retina. When we look at near objects, however, by the contraction 

 of certain muscles the pressure of these ligaments is eased and the 

 lens is able to approach its natural shape, thus altering the focus. 

 This power of accommodation of the eye is surely of some service in 

 judging distance ; it becomes noticeably lessened with advancing age. 



Distance- judging is effected by one eye partly in this way, by 

 the two eyes in conjunction, partly by judgment of the amount of 

 deviation from the parallel between the a.xes of the eyes — up to 

 the point where it requires violent internal squinting to see with 

 both eyes a smut on the tip of the nose — and partly by subtle 

 appreciation of the differences between the images on the two 

 retincT. But experiment shows that in ordinary life rapid, experienced 

 judgments of the meaning of the apparent size of objects, of relative 

 movement, of light and shade, and of perspective play a very great 

 part in our judgment of distance. 



The retina, the sensitive membrane itself, is very complex in 

 structure, despite its thinness. The light which falls upon it has to 

 traverse a network of nerve-cells and nerve-fibres before it reaches 

 the layer which we know to be the receptive surface. This layer is 

 known, from the shape of the cells of which it consists, as the layer 

 of the rods and cones; and a careful weighing of indirect evidence 

 enables us to form a theory of the meaning of these structures. It 

 is supposed that the cones are the cells which receive images in 

 bright lights, and that they alone have the power of discriminating 

 colours; while the rods are used in twilight vision, for seeing in very 

 dim lights, and are colour-blind. In some way associated with the 

 rods is a pigment, the "visual purple", which is bleached by light. 

 It is supposed that in night vision this bleaching of the visual 

 purple means the production of some new chemical substance from 

 the pigment, some substance which acts upon the rods and stimu- 

 lates them to send impulses to the brain. We must almost inevitably 

 assume the existence of some other substance which in a similar 

 way stimulates the cones, a temporary translation of light-energy 



