Ol'TICS. 



ol5 



\earsaregenerallysubject; 2. when the crystalline 

 cannot be drawn sufficiently near to the retina, re- 

 mote objects appear indistinct; this is the defect 

 under which short-sighted people labour. In each of 

 these cases, the images of the different points in the 

 object would be diffused over small circles on the 

 retina, and so, being intermixed and confounded with 

 each other, would then form a very confused picture 

 of the object. For, in the former case, the image ol 

 any point would be formed behind the retina, as the 

 refraction of the eye is not sufficiently strong to bring 

 the rnys (diverging so much as they do in proceeding 

 from a near point) to a focus at the retina. This de- 

 fect will therefore be remedied by a convex glass, 

 which makes the point whence the rays now proceed 

 more distant than the object; therefore the rnys, 

 falling on the eye, will now diverge less than before, 

 or else be parallel, and will, of course, be brought to 

 a nearer focus, viz., at the retina. In the latter 

 case, the image is formed before the retina, because 

 the refractive power of the eye is too great to permit 

 rays so little diverging (as they do in proceeding 

 from a distant point) to reach the retina, before they 

 are collected into a focus : in this case, the defect is 

 supplied by a concave glass, which makes the point 

 whence the rays diverge nearer than the object ; 

 consequently, the rays falling on the eye will now 

 diverge more than before, so as, when refracted 

 through the humours, not to come to their focus 

 before they reach the retina. Therefore spectacles 

 are constructed concave for short-sighted, and con- 

 vex for long-sighted people. And the frames of 

 spectacles should be so bent that the axis of both 

 glasses may be directed to the same point, at such a 

 distance as you generally look with spectacles. By 

 this means the eyes will fall perpendicularly upon 

 both glasses, and make the object appear distinct ; 

 whereas, if they fall obliquely upon the glasses, the 

 object will appear confused, and indistinct. 



Cause of Squinting. A person is said to squint, 

 when botli eyes do not seem to be directed to the 

 object at which he is looking. When either of the 

 eyes has a less perfect vision, or a different focal 

 length, or when there is any weakness in its external 

 muscles, we are apt to make use only of the good 

 eye ; and when we acquire the habit of doing this, 

 the imperfect eye is left at rest, and will sometimes 

 cease even to follow the movements of the other. 

 If the good eye is shut, and the bad one forced to 

 exert itself, the iris will be placed symmetrically 

 between the eyelids, and the squint formerly seen in 

 the eye will disappear. Should the eye, in this case, 

 still squint, the cause of it must be sought either in 

 the central hole of the retina not being at the extre- 

 mity of the axis, or in some mal-conformation, by 

 which the retina is not perpendicular to the axis of 

 the eye, at the point where they meet. This disease 

 of the eye, which is so generally neglected, might be 

 frequently cured, even in adults, by those who are 

 thoroughly acquainted with the structure and func- 

 tions ot this organ. 



The structure of the eye has already been de- 

 scribed in our article Eye (q. v.). In order that 

 distinct vision should be effected, it is necessary that 

 the rays from the luminous object should be con- 

 verged in points upon the retina. When, from the 

 little convexity in the crystalline lens, or any other 

 cause, these rays are not converged upon the retina, 

 but behind, indistinct vision will be the result. 

 When this is the case, a convex lens is interposed 

 between the object and the eye, to assist the humours 

 in converging the rays sooner, so that they may meet 

 on the retina. If the cornea, or the crystalline lens, 

 or both, be too flat, so as not to converge the rays 

 from the object c, fig. 15, until they arrive at the 



point F, which is beyond the retina, a convex lens 

 A B must be interposed between the object and the 

 eye, to assist the imperfect visual organ, and cause 

 the rays to converge at the point d, exactly on the 

 retina. In the other case, when the humours of the 

 eye have too much convexity, a plano-convex, or 

 double convex lens A B, fig. 16, is necess ry to 

 give divergence to the rays proceeding trora the 

 object at c, and cause them to converge on the 

 retina, instead of at the point F before they 

 reach it. 



The degree of curvature of the lenses must ob- 

 viously be adapted to compensate exactly for the 

 defect or excess of the curvature of the cornea 

 or crystalline lens. 



Accidental Colours.' One of the most curious affec- 

 tions of the eye is that which gives rise to ocular 

 spectra, or accidental colours. If we place a red 

 wafer on a sheet of white paper, and, closing one 

 eye, keep the other directed for some time to the 

 centre of the wafer, then, if we turn the same eye to 

 another part of the paper, we shall see a green wa- 

 fer, the colour of which will grow fainter and fainter 

 as we continue to look at it. This green image of 

 the wafer is called an ocular spectrum, or the acci- 

 dental, or opposite colour of red. By using different 

 coloured wafers, we obtain the following results : 



Colour of the Wafer. 

 Black, 

 White, . 



lU'd. 



Orange, 

 Yellow, 

 Green, . 

 Blue, 

 Indigo, . 

 Violet, 



Colour of the Spectrum. 

 White. 

 Black. 



Bluish green. 

 Blue. 

 Indigo. 



Violet, with a little red. 

 Orauge red. 

 Orange yellow. 

 Bluish green. 



When a strong impression of white light is made 

 upon the eye, a succession of remarkable spectra is 

 visible. When the sun was near the horizon, M. 

 ^Epinus fixed his eye steadily upon it for fifteen 

 seconds. Upon shutting his eye he saw an irregular, 

 pale, greenish-yellow image of the sun, surrounded 

 with a faint red border. When he opened his eye, 

 and turned it to a white ground, the image of the sun 

 was brownish-red, and its border sky-blue. With his 

 eye again shut, the image appeared green, and the 

 border a red, different from the last. On opening his 

 eye, and turning it to a white ground, as before, the 

 image was more red than formerly, and the border a 

 brighter sky-blue. His eye being again shut, the 

 image was green, approaching to sky-blue, and the 

 border a red, still differing from the former. When 

 his eye was again opened upon a white ground, the 

 image was still red, and its border sky-blue, but with 

 different shades from the last. At the end of four or 

 five minutes, when his eye was shut, the image was 

 a fine sky-blue, and the border a brilliant red ; and, 

 upon opening his eye, as formerly, upon a white 

 ground, the image was a brilliant red, and the border 

 a fine sky-blue. Experiments of a similar kind were 

 made by doctor Brewster, by looking at a brilliant 

 image of the sun's disk, formed by a concave mirror. 

 With his right eye tied up, he viewed this luminous 

 disk with the left, through a blackened tube, to pre- 

 vent any extraneous light from falling upon the reti- 

 na. When the retina was highly excited by this 

 intense light, he turned his left eye to a white ground, 

 and perceived the following spectra, by alternately 

 opening and shutting his eye : 



Spectra with the Irjl Eye open. 



1. Pink, surrounded with.green, 



2. Orange, mixed with pink, 



3. Yellowish brown, 



+. Yellow 



!>. Pure red Sky-blue. 



(t. Orange, Indigo. 



Speftn tritt 0* 

 left Bye thai. 



Green. 

 Blue. 



Bluish pink. 

 Lighter blno. 



