950 A MANUAL OF PHYSIOLOGY 



equally difficult to reconcile some of the phenomena of colour- 

 blindness with the Young-Helmholtz theory. Anomalies and defects 

 of colour-sensation are common accompaniments of pathological 

 lesions of the visual apparatus, and can be produced by various drugs, 

 as by abuse of tobacco. But colour-blindness, in its true sense, is 

 congenital, often hereditary ; the colour-blind are ' born, not made.' 

 And although the condition cannot be cured, it is of great importance 

 that it should be recognised in the case of persons occupying posi- 

 tions 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 sensations 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 (Practical Exercises, p. 998). 



Irradiation. The phenomenon known as 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. 417) appears, 

 in a good light, to be larger than an exactly equal black circle on a 

 white ground. The explanation is as follows : Owing to the aberra- 

 tion of the refractive media of 

 the eye (p. 912), all the rays 

 proceeding from the luminous 

 object are not brought accu- 

 rately to a focus on the retina, 

 and the image is surrounded 

 by diffusion circles (p. 912) 

 which encroach upon the un- 

 illuminated boundary. Physi- 

 FIG. 417. cally 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. 



Visibility of Radium and Rbntgen Rays. It is a question of interest 

 whether the retina can be excited by any other disturbances in the 

 ether than ordinary light waves. The Rontgen rays seem to be 

 capable of exciting visual sensations, and it is certain that this is true 

 of the rays or the emanation given off by radium. If in the dark a 

 vessel containing a radium salt is brought into the neighbourhood 

 of the closed eyelids or the temple, a sensation of brightness is ex- 

 perienced. This, however, is not due to direct excitation of the 

 retina by the radium rays, but to the phosphorescence set up in 

 the media of the eye by the radium emanation. Of all the tissues 

 the lens is most strongly phosphorescent after exposure to radium. 

 In blindness due to opacity of the media, where the retina is still 

 sensitive, the radium action is perceived, but not where the retina 

 is totally insensitive to ordinary light. The radium rays do not 

 cause bleaching of the visual purple 



