THE ACTION OF LIGHT 575 



THE PHOTO-CHEMISTRY OF THE RETINA 



It may be useful to refer back for a moment to the application of photo- 

 chemical facts to the retinal process. There seems no doubt that the visual 

 purple is an optical sensitiser ; it is bleached by light as these are ; and it absorbs 

 nearly the whole length of the visible spectrum. Whether the products of its 

 change are themselves capable of stimulating the light-receptors, or whether they 

 act catalytically in bringing about other changes in these receptors, is unknown. 

 It seems clear also that the peculiar form of the rods and cones must have some 

 significance, but the difficulty of the problem is obvious, and in the present state 

 of knowledge, speculation is of little use. 



THE "CHROMATIC FUNCTION" 



The power which certain animals have of modifying the colour of their skins 

 to match that of their surroundings is well known. The case of the chameleon is 

 often quoted. The manner in which it is effected is by the contraction or 

 expansion of pigment cells of various colours in the skin. To this is sometimes 

 added the iridescence due to interference of waves by means of a layer of fine 

 crystalline structure. The work of Brlicke (1851) on the chameleon was, after 

 that of Pouchet (1848), the first experimental investigation of the question. 

 Pouchet's work was on the frog and fish, and he introduced the name " chromatic 

 function" to express the dependence of the adaptation on the nervous system, 

 through the eye. Biedermann (1892) devoted much attention to the phenomena 

 in the frog. The skin epithelium cells of this animal contain yellow granules and 

 a deeper layer of crystalline particles, which show interference colours. In the 

 subjacent corium, there are a set of black pigment cells. The sciatic nerve 

 contains motor fibres for the latter cells and the chief centre appears to be in the 

 optic lobes with subsidiary centres in the spinal cord. There is also a nerve supply 

 to the chromatophores, by nerves accompanying the blood vessels. In the frog, 

 the receptors for the reflex arc are in the skin, chiefly that of the discs of the toes. 

 These are affected by the different qualities of the surfaces, which, in the habitual 

 surroundings of the animal, are associated with definite colours, such as stone, grass, 

 etc. When the toes are made anaesthetic, all colour adaptation disappears. The 

 eyes play no part in the phenomenon. On the other hand, the adaptation to details, 

 investigated by Sumner (1911) in Rhomboidichthys podas, a small flat-fish, allied 

 to the turbot, are effected by means of eye receptors. The photographs in Figs. 181 

 and 182 will give an idea of the range of the adjustment. It is to be remembered 

 that in reality the adaptation is more perfect than the monochrome reproduction 

 indicates, since it took place to varying shades of brown, as well as to black and 

 white. The change, when removed to a different background, was, in some cases, 

 obvious after a few seconds. When the fish were made blind, the chromatophores 

 went into their state of rest and no further adaptive reaction was possible. 



The use of this mechanism to its possessor seems to be twofold. The animal is 

 rendered invisible both to its enemies and also to the smaller fish which serve as 

 its prey. 



RADIO-ACTIVE PHENOMENA 



Although the effects of radium and similar elements which give off charged 

 particles, are not, strictly speaking, those of light, they may for convenience 

 be mentioned here. 



It was shown by Hardy that the negatively charged /^-particles of radium 

 produce coagulation of oppositely charged colloids, and the effect appears to be an 

 electrical one. 



The effects produced by radium on living tissues are very similar to those of 

 intense ultra-violet light, but more powerful. They have been used in therapeutics 

 for similar purposes. The explanation of their action is not yet clear. The 

 advantage of radium as regards ease of application to the spot required is obvious. 

 For further information on the extremely interesting phenomena of radio-activity, 



