OPTICS. 



43 



the eye, and, consequently, the image 

 of that point falls upon the central hole 

 in the retina. Every other point of the 

 same object is seen indistinctly, and the 

 indistinctness increases with the dis- 

 tance of the point from that which is 

 seen distinctly. The only perfectly dis- 

 tinct point of vision, therefore, is that 

 where there is no retina ; but we are 

 not entitled to ascribe this to the ab- 

 sence of the nervous matter, as the gra- 

 dual increase of distinctness towards 

 the central hole does not appear to be 

 accompanied with a gradual diminution 

 in the thickness of the retina. 



4. Indistinctness of vision at the base 

 of the optic nerve. It was discovered 

 by M. Mariotte, that when the image of 

 any object fell upon the base of the op- 

 tic nerve, the object disappeared. In 

 order to prove this experimentally, fix 

 on the side of a room, and at the height 

 of the eye, three wafers, two feet dis- 

 tant. Stand opposite to the middle 

 wafer with one eye shut, and, beginning 

 near the wall, retire gradually from it, 

 (looking always at the outside wafer, 

 which is on the same hand as the co- 

 vered eye,) till the middle wafer disap- 

 pears. " This will be found to take place 

 at about five times the distance at which 

 the wafers are placed, and when it does 

 happen, the other waters will be plainly 

 seen. If we use candles in place of 

 wafers, the middle one will not disap- 

 pear, but it will become a cloudy mass 

 of light. The base of the optic nerve, 

 therefore, is not insensible to light, it is 

 only unfit for giving distinct vision of 

 those objects whose images fall upon 

 it. M. Le Cat considered the size of this 

 portion of the retina to be about one- 

 third or one -fourth of a line ; but Daniel 

 Bernoulli found it to be about one- 

 seventh part of the diameter of the eye. 



5. Intermission in the vision of ob- 

 jects seen obliquely. The inability of 



the eye to preserve a sustained vision of 

 objects seen obliquely, was discovered by 

 Dr. Brewster. If when one eye is shut 

 we fix the other upon any point, such 

 as the head of a pin, A, stuck into a green 

 cloth, and continue 



Fig. 44. 



for some time look- 

 ing at the pin head, 

 objects, such as a 

 pen, B C, or a strip 

 of paper laid upon 

 the green cloth at 

 a distance from 

 the pin, will occa- 

 sionally disappear 



altogether ; the impression of the green 

 cloth extending itself over the part of 

 the retina on which the image of the 

 pen, B C, or strip of paper, was 

 formed. In a short time the vanished 

 object will again reappear. The same 

 effect is produced when both eyes are 

 used ; and when the object is highly 

 luminous, like a candle, it does not 

 wholly disappear, but expands itself into 

 a mass of nebulous light, which is of a 

 blue colour, encircled with a bright ring 

 of yellow light. 



But though objects thus seen indi- 

 rectly, or obliquely, occasionally dis- 

 appear, very minute objects, which oan- 

 not be seen by direct vision, may be 

 rendered visible by looking a little from 

 them. This has been observed by se- 

 veral astronomers, both with regard to 

 faint stars and to the satellites of Sa- 

 turn. When the eye is turned full upon 

 the star or satellite, it disappears ; but 

 when it is directed to another part of 

 the field of the telescope, the luminous 

 point will become distinctly visible. 

 This superior vision of a small point of 

 light seen obliquely, seems to arise part- 

 ly from the expansion of its image by 

 indirect vision, which makes its light 

 act upon a greater portion of the retina. 



6. Insensibility of the eye to direct 

 impressions of very faint light. When 

 the eye is steadily directed to objects 

 illuminated by a feeble gleam of light, 

 it is thrown into a state nearly as pain- 

 ful as that which is produced by an 

 excess of light. A. kind of remission 

 takes place in the conveyance of the im- 

 pressions along the nervous membrane : 

 the object actually disappears, and the 

 eye is agitated by the recurrence of im- 

 pressions which are too feeble for the 

 performance of its functions. 



" The preceding facts," ($ 5, 6,) says 

 Dr. Brewster,* " respecting the affec- 

 tions of the retina, while they throw 

 considerable light on the functions of 

 that membrane, may serve to explain 

 some of those phenomena of the disap- 

 pearance and reappearance of objects, 

 and of the change of shape of inanimate 

 objects, which have been ascribed by 

 the vulgar to supernatural causes, and 

 by philosophers to the activity of the 

 imagination. If in a dark night, for 

 example, we unexpectedly obtain a 

 glimpse of any object, either in motion 

 or at rest, we are naturally anxious to 

 ascertain what it is, and our curiosity 

 calls forth all our powers of vision, 



* Edinburgh Journal of Science, No. vi. p. 290. 



