VISION 1009 



dark. This is even true when it has been kept in the dark in a saturated 

 solution of sodium chloride, and is then, after washing with physio- 

 logical salt solution, bleached by light. Here the regeneration of the 

 pigment cannot be the result of vital processes, but must be due to 

 chemical changes in products formed from the original pigment by the 

 action of light. No such regeneration takes place in a retina which, 

 after having been bleached in situ, is removed without the pigmented 

 epithelium and placed in the dark; and the only probable explanation 

 of the difference is that in this case the photo-chemical substances 

 from which visual purple can be formed have been absorbed into the 

 circulation, and have so escaped. 



The inner segments of the cones of certain animals (birds, reptiles, 

 amphibia, and some fishes) contain globules of various colours, ranging 

 over almost the whole spectrum, and including, besides, the non-spectral 

 colour, purple. The globules are composed chiefly of fat with the 

 pigments (chromophanes, as they have been called) dissolved in it. 

 The function of these globules is unknown. They cannot be concerned 

 in colour vision, or, at least, they cannot be essential to it, for in the 

 human retina they do not exist. 



The yellow pigment of the macula lutea does not belong to the layer 

 of rods and cones; it only exists in the external molecular layer and the 

 layers in front of it; in the fovea centralis it is absent. 



Time necessary for Excitation of the Retina by Light Fusion of 

 Stimuli. Whatever the exact nature of retinal excitation may be, 

 it is called forth by exceedingly slight stimuli. A lightning flash, 



although it may last only - th of a second, lasts long enough 



3 1,000,000 



to be seen. A beam of light thrown from a rotating mirror on the. 



eye stimulates when it only acts for fi th of a second. The 



8,000,000 



minimum stimulus in the form of green light corresponds, as we have 

 already seen (p. 758), to a quantity of work equivalent to no more 



than ^ erg that is, about ^Q gramme -millimetre, or ^ 7 milli- 

 gramme-millimetre, which is the work done by th of a 



10,000,000 



milligramme in falling through a millimetre; and it cannot be doubted 

 that a portion even of this Lilliputian bombardment is wasted as heat. 

 So quickly, too, is the stimulus followed by the response that no latent 

 period has as yet ever been measured. It is certain, however, that 

 there is a latent period, as surely as there is a latent period in the 

 excitation of a naked nerve-trunk, although this also has never been 

 experimentally detected. The analogies, in fact, between a muscular 

 contraction and a retinal excitation are numerous and close. Like the 

 muscle, the retina seems to possess a store of explosive material which 

 the stimulus serves only to fire off. The retina, like the muscle, is 

 exhausted by its activity, and recovers during rest. Like the muscle 

 curve, the curve of retinal excitation rises not abruptly, but with a 

 measurable slowness to its height, and when stimulation is stopped, 

 takes a sensible time to fall again, the retinal impression outlasting the 

 luminous stimulus by about one-eighth of a second. With compara- 

 tively slow intermittent stimuli the retinal, like the muscle curve, 

 flickers up and down. When the rate of stimulation is increased, the 

 steady contraction of the tetanized muscle is analogous to the fusion 

 of the individual stimuli by the tetanized retina (or retino-cerebral 

 apparatus) into a continuous sensation of light, such, e.g., as the bright 



6 4 



