VISION. 



1439 



combination of crown and flint glass. He 

 supposed the mean refractive power of flint 

 glass, as compared to crown glass, to be 158 

 to 153 ; Fraunhofer states it to be 164 to 

 153. The prismatic dispersion in English 

 flint glass is l times, but in Fraiinhofer's, it 

 is twice as great as in crown glass. 



Such are the chief facts concerning light, 

 which bear reference to vision : to the con- 

 sideration of the physiology of which we now 

 proceed. 



Phenomena of Vision. The special function 

 with which the retina is endowed being the 

 perception of light, a marvellous range of 

 phenomena is open to the inquirer. It is 

 , indeed a wonderful thing to have ascertained 

 * beyond doubt, that in perceiving the tint of 

 the scarlet geranium our eyes are affected by 

 undulations recurring four hundred and eighty- 

 two millions of millions of times in a second ; 

 that before we can appreciate the tint of the 

 yellow blossoms of the gorse or laburnum, 

 five hundred and forty-two millions of mil- 

 lions of vibrations must have taken place ; 

 and that to discriminate the colour of the 

 violet, not less than seven hundred and seven 

 millions of millions of movements must have 

 been communicated to the fibrillae of our re- 

 tinae ! Whilst such facts almost transcend 

 the powers of human conception, their esta- 

 blishment is a striking triumph of human in- 

 tellect. But how great ought to be our 

 admiration of that Omnipotence which has 

 endowed the eye with the gift, not merely of 

 appreciating one colour, but of distinguishing, 

 in all their shades, the inexpressibly compli- 

 cated vibrations which mark the hues of a 

 parterre of flowers, and characterise the gor- 

 geous plumage of the birds which give anima- 

 tion to a tropical forest. The sense of sight, 

 in its ordinary acceptation, may be defined as 

 the recognition by the mind of certain im- 

 pressions made upon the retina, and commu- 

 nicated through the medium of the optic 

 nerve to the encephalon ; a sound condition 

 of each and all of these parts (which may be 

 considered as the media of communication, so 

 far as one sense is concerned, between the 

 external world and the mind) is indispensable 

 for perfect vision. Light may fall upon the 

 retina, and the images of objects may be there 

 depicted, but should the optic nerve be un- 

 sound, or certain portions of the brain be 

 disorganised, no responsive image is called up 

 before the mind ; the eye may gaze upon the 

 noonday sun, but all is dark within. 



The natural stimulus of the retina is the 

 luminous rays : the appreciation of light and 

 colour its active condition ; and its state of 

 repose suggests the appearance of darkness : 

 but besides light, any other excitement ap- 

 plied to the retina or optic nerve gives rise to 

 the same result, the production of luminous 

 appearances. Pressure upon the eye-ball, the 

 electric current, or vascular congestion, all 

 excite this special phenomenon. Occasion- 

 ally, too, irritation of the brain has the same 

 effect ; and many are the waves and corrus- 

 cations, the fiery clouds and flaming spectra, 



which haunt the amaurotic when certain 

 morbid complications exist. The phantasms 

 of fever, and the illusions of the dyin<;, are 

 to be placed in the same category with the 

 above. 



For visual purposes, a certain amount of 

 stimulation of the retina is necessary ; hence 

 it is both difficult and painful to discern ob- 

 jects in very faint light : but in this respect 

 the retina is endowed with great powers of 

 accommodation. It is well known that per- 

 sons long immured in dungeons profoundly 

 dark to ordinary eyes, have acquired the 

 power of discerning the most minute objects. 

 On the other hand, excessive light is in- 

 jurious, for by it vision may be instantane~ 

 ously extinguished, as happens by a stroke of 

 lightning. The accommodation to various 

 amounts of light is, however, gradual, as is 

 proved by passing from a dark room into sun- 

 light, or into a brilliantly lighted apartment. 

 In this case the pupil has been widely dilated 

 to admit the greatest possible quantity of light 

 into the eye. For the first second or two 

 after arriving in the light, all is confusion; and 

 there is even pain, from the flood of light 

 which thus breaks in on the retina through 

 the widely dilated pupil, and takes it, as it 

 were, unawares. On the other hand, a per- 

 son entering a moderately dark room from 

 full sunlight, sees nothing for a time, because 

 the strongly contracted pupil admits so little 

 light, that no sensation is produced. After a 

 while the pupil dilates, and vision gradually 

 becomes complete. In order that a clear idea 

 may be formed of the mode in which images 

 are depicted upon the retina, and also of the 

 causes of myopia and presbyopia, it is neces- 

 sary that the mode of action of convex lenses 

 should be described. 



Fig. 884. 



Let c D be a biconvex lens, and A B an ob- 

 ject on one side of it, but further removed 

 than the focal point F. Every point of 

 the object will send forth rays in all direc- 

 tions, but to avoid confusion, we will only 

 consider those flowing from the extremities 

 AB. The rays emitted from A undergo re- 

 fraction, are altered in their direction, and 

 are united at A' upon the secondary axes 

 drawn from A through G, the centre of the 

 lens. The rays from B are in like manner 

 united at B'; consequently, A'B' is the image 

 of the object AB, but inverted. Seen from 

 the middle of the lens, the image and object 

 appear at the same angle, for the angle B'GA' 

 is equal to the angle BGA, being angles at th6 

 vertex. The relative size of the object or the 

 image depends upon the relative distance of 



