December, igo6.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



605 



the pectoral fins of flying fish .... one thirty- 

 second," and the muscles of a nearly related non-fly- 

 ing fish only one hundred and fifty-fourth. 



" As before, it does not prove that bats or flying fish 

 flap or do not flap their wings, but it gives a different 

 and, I hope, a proper aspect to the figures which have 

 done duty — of a kind — for so many years.'' 



Light and the Visual 

 Sense. 



A Study in Biological Physics. 



Bv Henrv a. Fotherbv, D.I^h. (Camb.), 

 L.R.C.P.(Lond.), &c. 



" The method by which a ray of light is able to stimu- 

 late the endings of the optic nerve in the retina in such 

 a manner that the visual sensation is perceived by the 

 cerebrum is not yet understood. It is supposed that 

 the change effected by the agency of the light which 

 falls upon the retina is, in fact, a chemical alteration in 

 the protoplasm, and that this change stimulates the 

 optic nerve endings." — (Halliburton.) 



We know that the energy of light as well as heat 

 and electricity are capable of producing chemical action. 

 For instance, if a mixture of chlorine and hydrogen 

 gases, which will keep indefinitely in the dark, is ex- 

 posed to sunlight they will combine with explosive 

 violence. The principle on which photography depends 

 is the influence of light in producing chemical change in 

 silver chloride, which becomes blackened owing to the 

 reduction of silver. It is from the radiant energy of 

 sunlight that chlorophyll, the green colouring matter of 

 plants, derives chemical energy whereby plants are 

 enabled to build up their tissues from the elements of 

 carbon dioxide and water. 



It has Ijecn observed that movements take place in 

 the pigment granules of the retinal cells under the in- 

 fluence of light. The retinal cones also shorten in its 

 presence and elongate in its absence. In the retinal 

 rods of certain animals, notably frogs, there is a cer- 

 tain pigment called visual purple, which, though present 

 in the dark, disappears in the presence of light, and 

 reappears again directly the light is withdrawn. The 

 \ isual purple is also foimd to undergo distinct changes 

 of colour when exposed to other lights than white light. 



It was on observations such as these that Hering 

 based his theory of colour vision, consisting of six 

 primary colour sensations, in opposition to the Young- 

 Ilelmholtz theory of three only. The former suggests 

 that these consist of three pairs of antagonistic or com- 

 plimental colour sensations, black and white, red and 

 green, yellow and blue, and that the stimulus producing 

 each severally is caused by changes either of disintegra- 

 tion or assimilation taking place in certain three sub- 

 stances of the nature of visual purple, which it is 

 assumed exists in the retina. Thus, in the case of the 

 red-green substance, if assimilation is in excess of dis- 

 integration the sensation is red, if the reverse it is 

 green, but when equal no sensation occurs. The 

 Young-Hclmholtz theory, on the other hand, teaches 

 that there arc only three primary elementary colour 

 sensations — red, green, violet — and that the end-organs 

 of the optic nerve in the retina, the rods and cones, 

 consist of three varieties, each one specialised to re- 

 spond to one of these three colour sensations, and that 

 all the different shades of colour are due to the different 



degrees in which they are severallj' excited, the sensa- 

 tion of white being produced only when they are equally 

 stimulated. This latter is the more generally accepted 

 theory of colour vision. How these three colours, red, 

 green, and violet, were selected and found to be the only 

 fundamental colour sensations would take up too much 

 space to describe here. It must suffice to say that it 

 was due to a series of experiments by which the retina 

 was exhausted for various colours, with the result that 

 the fatigue manifested itself in these three colours, and 

 it was found also that these three colour sensations 

 could not be produced by any combination of other 

 colour sensations. 



The eye is not only a complicated optical instrument, 

 but it is at the same time a vital organ of extreme 

 delicacy functioned for the purpose of receiving sensory 

 impressions of radiant energy, the velocity of whose 

 radiations lie between certain definite limits, and trans- 

 mitting them to the optic nerve centres in such a way 

 that they can be translated into sensations of light and 

 colour. It is owing to this fact that these rays have 

 received the name of luminiferous ether. When the 

 ether vibrations have acquired a velocity of 450 millions 

 of millions a second they affect the retina and become 

 appreciated as the sense of red; as the vibrations in- 

 crease in velocity the colour sensations experienced are 

 those of the solar spectrum from left to right, namely, 

 from red through orange, yellow, green, blue, purple 

 to violet, at which point the ether vibrations have 

 reached the enormous velocity of 727 millions of millions 

 per second. Beyond this point, what is technically 

 called the ultra-violet portion of the spectrum is reached, 

 where the vibrations become too rapid to be appreciated 

 by the eye, and they consequently cease to be luminous. 

 For example, " When a wire is heated in a spirit lamp 

 placed in a dark room the particles of which the wire is 

 composed are thrown into a state of violent vibration. 

 As the heat increases the vibrations increase in rapidity. 

 They are communicated to the ether, which surrounds 

 and permeates e\erything; and the movements thus 

 set up — infinitely small waves in this infinitely big ocean 

 which fills all space — are sent off on their journey in all 

 directions. At first the undulations are too slow to 

 affect the retina, though they affect the skin. W'e per- 

 ceive that the wire is hot if we hold it to our cheek an 

 inch or two away, but our eye reveals no change. As 

 the heat increases the rate of the waves increases, and 

 when they reach to the enormous number of about 450 

 billions — that is, 450 millions of millions — per second, 

 we see that the wire is glowing red. The ordinary 

 physical cause of sight, then, is found in the fact that 

 undulations or vibrations of almost inconceivable 

 rapidity are affecting an organ specially adapted for 

 receiving them, viz., the retina. ... If we think 

 this over we shall see that it involves the conclusion 

 that what we call light does not exist in the universe 

 apart from eyes to sec it. The ' light rays ' that physi- 

 cal science deals with are, in themselves, no more red 

 or blue than the dark heat rays or than the X-rays of 

 which we have heard so much of late; the sunshine 

 would have no splendour, but from the eyes which see 

 it. If eves did not exist, the sun's rays would produce 

 their beneficicnt effects on plants and animals just as 

 thev do now, but the splendour and beauty would not 

 exist. Thev are due, not to the physical cause, but to 

 the mysteries of a piece of living tissue, the retina, 

 which has been given the power to select those rays 

 composed of imdulations of a certain degree of rapidity, 

 and to somehow make them the occasion of mental 

 facts of unspeakable beauty." — (Ryland. " The Story 

 of Thought and Feeling.") 



