THE RETINAL CURRENT. 97 



phores ; and we may now venture to attempt to investigate 

 how it is that an adaptation to the colour of surrounding 

 objecls can be effected by these variations in the colour of the 

 skin, that are only indirectly dependent on the light. 



Professor Dewar has recently shown 28 that the different 

 colours of the spectrum influence the eye and the retina in very 

 different ways by producing an electric current which has been 

 termed the ' optic current.' The intensity of this current, ac- 

 cording to Dewar, is greatest under yellow light, weakest under 

 purple light, and nil in total darkness. Of course we cannot 

 directly compare the stimulus which is communicated from the 

 rays of light through the optic nerves to the sympathetic nerve, 

 and then by way of the spinal nerves to the nerves of the skin, 

 and finally to the chromatophores, with this ' optic or retinal 

 current,' because an electric current invariably takes the shortest 

 road, which the nervous irritation above described certainly does 

 not. But if we assume that the measure of the force exercised 

 by the eye on the chromatophores may bs approximately esti- 

 mated by the force of the retinal current, an explanation of the 

 phenomena of tb.3 chromatic function would be easily found. 



Every object reflects the light according to the nature of its 

 colour ; black surfaces, when they are not too smooth, absorb 

 the rays in the highest degree, red come next in order, and then 

 yellow. White reflects nearly all the rays ; hence a black back- 

 ground, reflecting but little light, will stimulate the eye in a 

 very faint degree, and the excitation, analogously to the ascer- 

 tained working of the retinal current, will apparently not be 

 strong enough to occasion the contraction of the black chromato- 

 phores ; these remain expanded, and give the skin a dark hue. 

 If the light is reflected from a red or blue object, the somewhat 

 stronger stimulation causes the black or brown chromatophores 

 to contract while it does not affect the red or yellow ones ; the 

 animal then exhibits a reddish or bluish tint. The light 

 reflected from green or yellow bodies produces a still stronger 

 effect on the chromatophores, till a pure white light makes all 

 the inmost layer of the chromotaphores contract, and the ani- 

 mal is almost colourless. This explanation coincides perfectly 

 with Pouchet's observations, though Heincke certainly makes 



