280 



SENSE OF SIGHT. 



ferent degrees of obliquity, on the denser medium, B C ; all the 

 that are in fits of easy refraction will pass through the medium to 

 the point D ; whilst those that are in fits of easy reflection, will be 

 thrown back into the medium ABC; so that we may presume, that 

 all the rays, which fall upon the parts of the medium B C, correspond- 

 ing to the bases of the dark cones, will be reflected back, whilst those 

 that correspond to the bases of the light cones, will pass to a focus at 

 D. Now, if all the bundles of rays, trans- 

 mitted through the surface B C, be accurately 

 collected into a focus, no other consequence 

 will arise from the other bundles of rays hav- 

 ing been reflected back, than that the focus will 

 be less luminous than it would have been had 

 all the rays been transmitted through it. This 

 explains why, at the distance of distinct vision, 

 we have only a single impression made on the 

 eye. But if we approach the object A, so that 

 the focus is not thrown, say upon the screen 

 R T, which may be presumed to represent the 

 retina but behind it ; the dark and light 

 spaces will be represented upon the screen, 

 and, of course, in concentric circles. This hap- 

 pens to the eye, when the hair or .needle or 

 other object is brought nearer to it than the 

 visual point. We can thus understand, why 

 concentric circles, of the nature mentioned, 

 should be formed upon the retina ; but how is 

 it, that the objects seen preserve their linear 

 form ? Suppose a b, Fig. 126, to be a luminous 

 cone, which in a fit of easy refraction has im- 

 pinged upon the retina ; and A B, b a, the concentric circles, corre- 

 sponding to the rays that have been reflected. It is obvious, that every 

 point of the object will be the centre of so many concentric circles on 

 the retina ; and if we imagine the fits of easy reflection and refraction 

 to be the same around those points, we shall have the dark and lucid 

 lines represented by the tangents to these circles ; and hence we can 

 comprehend why, instead of having one lucid line e f, we have three, 

 separated by dark lines parallel to them ; and if the light from the 

 luminous point be strong enough to form more lucid rings than are re- 

 presented in Fig. 126, and the breadth of those rings be not too minute 

 to be perceived, we may have the appearance of five, seven, or more 

 lucid lines, separated by parallel dark lines. 



The undulatory theory of light offers another explanation of the 

 phenomenon of fits. The waves in the luminous ether along a ray of 

 light, may meet the surface of a transparent body in different conditions 

 of condensation or rarefaction, and their transmission or reflection may 

 be determined by these conditions. 



We proceed now to consider the advantages, which the mind derives 

 from the possession of this sense, so pre-eminently entitled to the epithet 



G c e E 



Multiple Vision with One Eye 



