THE SENSES 833 



dition cannot be cured, it is of great importance that it should be 

 recognised in the case of persons occupying positions such as 

 those of engine-drivers, railway-guards, and sailors, in which coloured 

 lights have to be distinguished. For, while it is true that the sensa- 

 tions which red and green lights give the colour-blind are far from 

 being identical (Pole) under favourable conditions, it is precisely 

 when the conditions are unfavourable, as in a fog or a snow-storm, 

 that the capacity of distinguishing them becomes invaluable. 



Irradiation was first described by Kepler, who gave as an example 

 the appearance known as the ' new moon in the old moon's arms,' 

 where the crescent of the new moon seems to overlap and embrace 

 the unilluminated portion of the lunar disc. A white circle on a 

 black ground (Fig. 320) appears, in a good light, to be larger than 

 an exactly equal black circle 

 on a white ground. The ex- 

 planation seems to be as fol- 

 lows : Owing to the aberration 

 of the refractive media of the 

 eye (p. 794) all the rays pro- 

 ceeding from the luminous ob- 

 ject are not brought accurately 

 to a focus on the retina, and Fio. 320. 



the image is surrounded by 



diffusion circles which encroach upon the unilluminated boundary. 

 Physically these represent a weaker illumination than that of the 

 image proper, and therefore the latter ought to stand out in its 

 real size as a brighter area surrounded by weaker haloes. That 

 this is not the case, and that the image is projected in its full 

 brightness for a certain distance over its dark boundary, is due to 

 the fact that the eye does not recognise very small differences of 

 brightness. When the accommodation is not perfect, the diffusion 

 circles are, of course, much wider, and irradiation is better marked 

 when the object is a little out of focus. 



The Movements of the Eyes. That the eyes may be efficient 

 instruments of vision, it is necessary that they should have 

 the power of moving independently of the head. An eye 

 which could not move, though certainly better than an eye 

 which could not see, would yet be as imperfect after its 

 kind as a ship which could run before the wind, but could 

 not tack. The mere fact that the angle between the visual 

 axes must be adapted to the distance of the object looked 

 at renders this obvious ; and the beauty of the intrinsic 

 mechanism of the eyeball has its fitting complement in the 

 precision, delicacy, and range of movement conferred upon 

 it by its extrinsic muscles. 



Nof only are movements of convergence and divergence 



53 



