THE SENSE OF SIGHT. 187 



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 seen? For tlii.s there arc several excellent means which we will briefly notice. 

 In the fir.-it place, all ether waves have the power of giving wartnth — that is to 

 say, of exciiiug, when they strike the nerves of touch, the sensation of warmth — 

 of causing matter to expand ; in short, of producing all the effects attributable 

 to heat. Jn other words, heat and light arc the same, both consisting in the 

 vibrations of that elastic medium known as the luminous ether. The power of 

 warming is not, however, proper to all vibrations of the ether in an equal de- 

 gree ; tlic vibrations which are too slow to excite the nerves of vision, and 

 which hence, in the spectrum, fall outside of or beyond the red, possess that 

 power in a high degree ; we recognize them by their action upon the apparatus 

 for measuring heat. In the second place, the rays of light have the jiower of 

 piuducing, by their action on certain substances, chemical decomposition; for 

 instance, of rendering black certain combinations of silver. This chemical 

 effect pertains in the highest degree to the most rapid vibrations of the ether, 

 and therefore to those invisible vibrations which foil beyond the violet ; we 

 recognize their presence by their chemical effect. They are to be recognized, 

 however, in other ways, being capable, for instance, of becoming visible, with a 

 blue color, when, by certain means which we possess, but which we cannot stop 

 to explain, their vibrations are rendered slower. Under favorable circumstances, 

 indeed, when enabled to operate with great intensity on the eye, they become, 

 even without abatement of frequency, directly visible ; that is to say, when they 

 take effect very strongly on the optic nerve they are capable of exciting it. 



Such, briefly, is the physical theory of the origin of sensations of color. 

 With this are further connected some important physiological considerations. 

 We have ])roved at large that light and color exist not externally, but are en- 

 tirely qualities of sensation, conditioned by the nature of the optic nerve and 

 its perceptive apparatus in the brain. A sensation of light, therefore, may be 

 determined not merely by the impact of a wave of the luminous ether on the 

 retina, but by any excitation of the optic nerve, of whatever kind it may be, 

 provided it strike the extremities of the nerve in the retina or its stem and the 

 current be evoked in its fibres. It has been already mentioned in passing that 

 the strongest irritant of all nerves, the electric current, produces, Avhen directed 

 through the eye, intense perceptions of light, or, to speak more exactly, a bright 

 flash at the moment of commencing and ceasing, a radiant yellow and violet 

 colored image dining its continuance. Pressure on the nerves of sight or their 

 division, sometimes required by morbid degeneration of the eyeball, also pro- 

 duces a sensation of light. Even mechanical irritation of the retina induces 

 the phenomenon. Every one may recollect from experience that a blow or 

 thrust against the eye is attended with an appeai'ance of sparks, but the fact 

 may be verified at any miiment by a more commodious experiment. If the e;ye 

 be closed and a finger end pressed against the lateral parts of the eyeball, and 

 therefore against the sclerotic coat under which lies the retina, a bright circle 

 will appear in the darkened field of vision ; not, howevei', at the point of pres- 

 sure, but in the opposite part, as, for instance, towards the nose when the pres- 

 sure is on the side next the temple, underneath when the pressure is from above. 

 We will inquire farther on, why this transposition of the perception should take 

 place. To adduce still another example of these luminoias apparitions through 

 mechanical irritation of the optic nerve: When from physical affections, spirit- 

 uous potations, or morbid excitement a determination of blood to the head takes 

 place, fine lummous specks are apt to glimmer before the eyes, the particles of 

 which float confusedly in the field of vision, and most distinctly when the sight 

 is directed to a bright sby or attention is paid to the phenomenon with the eyes 

 closed. This glimmering arises from over fulness of the blood vessels traversing 

 the retina itself; the loaded vessels and probably the single corpuscles of the 

 blood which permeates them exert a pressure on the elements of the retina lying 

 behind them and thereby produce the excitation of the optic ner\e. Once more : 



