490 PHYSIOLOGY 



been already considered, namely, the relationship between actinic, visual, and 

 heat rays, and the effects of colour mixture, there are others of importance 

 to vision. (1) The property of travelling in straight lines ; (2) of suffering 

 refraction ; (3) of causing chemical change. The first property can be easily demon- 

 strated by investigating shadow formation. But it should be noted that straight line 

 propagation is only approximate, for it can be shown that at the edge of a light ray 

 there may be considerable deviation. This effect is called diffraction, and will be con- 

 sidered in greater detail later. The second property, namely that of suffering refraction, 

 is found, to take place whenever light travels from a medium of one optical density 

 (refractive index) into that of another. Briefly, refraction consists of a deviation of 

 the light rays towards the normal to the surface, when entering a denser medium, and 

 away from the normal on entering a lighter. Rays of long wavelength tend to keep 

 their original direction more than those of short wavelength. Red rays are therefore 

 less refracted than orange rays, and orange less than yellow, and so on according to 

 wavelength. It is in this way that the spectrum is formed in the special apparatus 

 for experiments on colour mixture referred to above. But the most important effect 

 of refraction, from the point of view of vision, is the formation of an image by a lens. 

 This action may be briefly explained by considering what will happen to a beam of 

 light, when it encounters a mass of high optical density having a convex spherical 

 surface. Since the rays on entering are deviated towards the normal to the surface, 

 it is clear that rays that have entered near the edge of the lens will be bent towards one 

 another, and will therefore approach as they travel through the lens substance, till 

 they ultimately meet at the focus with all the other rays that have entered the lens from 

 the same source as themselves. But if there be a number of different sources, then the 

 rays from each are found to form their own focus, at a position that may be determined 

 either by experiment or by the rules of geometry. The positions of the different foci 

 are found to bear the same relationship to one another as those of the original sources. 

 or in other words an image is produced. This important subject will be found referred 

 to again in greater detail in section 5. 



PHOTO-CHEMICAL CHANGE, which is the third property of light mentioned 

 above, is well illustrated by photography. The most important principle of light act ion 

 is that light, to cause chemical change, must be absorbed (Draper's law). For example, 

 an ordinary photographic plate which is found to be opaque to blue violet and ultra 

 violet rays, and to be transparent to the rest of the visible spectrum, is therefore sensitive 

 to the former rays but inactive to the latter. Further by colouring the plate by a 

 dye which absorbs red, yellow and green, it is possible to make the plate react to these 

 rays. Draper's law is therefore obeyed. Chemical reactions caused by light are of 

 many types, but may be divided into reversible and irreversible. The former type 

 of reaction occurs only so long as the light acts (the change from CO to oxyhaemoglobin 

 may l>e given as an example), while the latter type remains in the final state that has 

 In -en reached (the changes in a photographic plate may be given as an example). There 

 is further another and more complicated type which, when once started by an incident 

 beam of light, goes on automatically with an evolution of energy until the reaction is 

 completed. These effects of light are probably of great importance in connection with 

 vision and will therefore receive further consideration later. 



THE MECHANISM OF VISION 



The organ of vision makes use of the properties of light which have been above de- 

 scribed, and \\c may briefly consider t he form t hat such an organ would fake. To com- 

 mence \\illi, there must be some method of causing light to stimulate (he end of a ner\e. 

 Une possible scheme \\oiild be to connect the nerve to a modified taste bud, which had 



been selected tor it s sensil i \ eliess to the presence of a chemical Sll bst anee called A. If 



this substance A is formed when light acts on another substance l>, so long as light is in- 

 cident A. is being formed and the end of thener\e is being stimulated. With cessation of 



the light howevel I', i ief.>im,-d from A, and the stimulus to the ner\eat <n , , .-.ises. The 



