44 



OPTICS. 



This anxiety, however, serves only to 

 baffle us in our-attempts. Excited by a 

 feeble illumination, the retina is not 

 capable of affording a permanent vision 

 of the object, and while we are straining 

 our eyes to discover its nature, it will 

 entirely disappear, and afterwards re- 

 appear and vanish alternately. The 

 same phenomenon may be observed in 

 daylight by the sportsman, when he en- 

 deavours to mark upon the monoto- 

 nous heath the particular spot where 

 moor-game has alighted. Availing him- 

 self of the slightest difference of tint in 

 the adjacent heath, he keeps his eye 

 steadily fixed upon it as he advances ; 

 but whenever the contrast of illumina- 

 tion is feeble, he invariably loses sight 

 of his mark ; and if the retina is again 

 capable of taking it up, it is only to lose 

 it a second time." 



7. Seat of vision. The inability of 

 the base of the optic nerve to perform 

 the same functions as the parts of the, 

 retina which surround it, led Mariott,e 

 to suppose that the choroid coat, which 

 lies immediately below the retina, is the 

 seat of vision. This opinion was con- 

 firmed by the fact of the transparency 

 of the retina, which rendered it unfit for 

 the reception of images, and by the 

 opacity of the choroid. 



In the eye of the cuttle-fish, Dr. 

 Knox* has shown that there is inter- 

 posed between the vitreous humour and 

 the retina an excessively dark and 

 opaque pigment of considerable consist- 

 ency, assuming the form of a mem- 

 brane. Hence, if the retina performs 

 any part in vision, the impressions 

 made by the images on this dark mem- 

 brane must be conveyed by vibration 

 to the retina behind it. In like manner 

 in the human eye, the impressions on 

 the choroid coat may be conveyed to 

 the retina before it, by the vibrations 

 of the choroid. This view of the matter, 

 which is however not without its diffi- 

 culties, reconciles the opposite senti- 

 ments which have been so long enter- 

 tained. The choroid coat has generally 

 been supposed to be black, and M. Le 

 Cat states that it grows less black with 

 age. I have however observed in young 

 persons, generally below the age of 

 twelve, that it reflects a brilliant crimson 

 colour, similar to what we observe in the 

 eyes of dogs and other animals. Hence 

 it would follow, that if the retina is 

 affected by rays which fall upon it, the 



* Edinburgh Journal of Science, No. vi. p. 199. 



young persons above mentioned ought to 

 see the crimson light reflected by the 

 choroid and falling upon the retina, in 

 its progress out of the eye. I have 

 ascertained, however, that' this is not 

 the case ; and consequently we obtain 

 a strong argument in favour of the 

 opinion, that the retina is affected only 

 by the vibrations communicated to it by 

 its contact with the choroid coat.* 



8. Duration of the impressions of light 

 upon the eye. Every person must have 

 observed that when we whirl in the hand 

 a burning stick, a circle of light is seen 

 marking out the paths described by its 

 burning end. As the burning extremity 

 can only be in one point of the path at 

 the same instant, it is manifest that the 

 impression of its light continues some 

 time on the eye. M. D'Arcy found that 

 the light of a live coal, placed at the dis- 

 tance of 165 feet, continued its impres- 

 sions on the eye during the seventh part 

 of a second. This affection of the eye 

 has been ingeniously used by Dr. Paris 

 in constructing a toy called the Thauma- 

 trope, from two Greek words which 

 signify to turn wonders. It is shown in 



Fig. 45. 



where AB is a circle cut out of card, 

 and having two silk strings, C D, fixed 

 to it, by twisting which with the finger 

 and thumb of each hand it may be 

 twirled round with considerable velocity. 

 On one side of the piece of card there 

 is drawn any object, such as a chariot, 

 and on the other side of it the charioteer 

 in the attitude of driving, so that when 

 the card is twirled round, we see the 

 charioteer driving the chariot, as in the 

 figure ; or, in consequence of the dura- 

 tion of the impressions of light on the 

 retina, we see at once what is drawn on 

 both sides of the card. 



* M. Lehot has recently endeavoured to show that 

 the seat of vision is anterior to the retina, and that 

 vision is effected by images of three dimensions 

 formed in the vitreous humour. He considers it 

 probable, that the sensation is conveyed to the retina 

 by small nervous filaments extending from the latter 

 into the vitreous humour. 



