SENSE OF VISION. 407 



5. Sense of Vision. 



533. The objects of this sense are bodies, which are either in themselves 

 luminous, or which become so by reflecting the light that proceeds from others. 

 "Whether their light is transmitted by the actual emission of rays, or by the 

 propagation of undulations analogous to those of sound, is a j^iestion at pre- 

 sent keenly debated amongst Natural Philosophers ; but it is of little conse- 

 quence to the Physiologist, which is the true solution ; since it is only with 

 the laws, which actually regulate the transmission of light, that he is concerned. 

 These laws it may be desirable here briefly to recapitulate. 



534. Every point of a luminous body sends off a number of rays, which 

 diverge in every direction, so as to form a cone, of which the luminous point 

 is the apex. So long as these rays pass through a medium of the same dens- 

 ity, they proceed in straight lines ; but, if they enter a medium of different 

 density, they are refracted or bent. towards the perpendicular to the surface 

 at the point at which they enter, if they pass from a rarer into a denser me- 

 dium, and from the perpendicular, when they pass from a denser medium 

 into a rarer. It is easily shown to be a result of this law, that, when parallel 

 rays passing through air fall upon a convex surface of glass, they will be 

 made to converge ; so as to meet at the opposite extremity of the diameter of 

 the circle, of which the curve forms part. If, instead of continuing in the 

 glass, they pass out again, through a second convex surface, of which the di- 

 rection is the reverse of the first, they will be made to converge still more, so 

 as to meet in the centre of curvature. Rays which are not parallel, but which 

 are diverging from a focus, are likewise made to converge to a point or focus ; 

 but this point will be more distant from the lens, in proportion as the object 

 is nearer to it, and the angle of divergence consequently greater. The rays 

 diverging from the several points of a luminous object, are thus brought to a 

 corresponding focus ; and the places of all these foci hold exactly the same re- 

 lation to each other, with that of the points from which the rays diverged; so 

 that a perfect image of the object is formed upon a screen held in the focus 

 of the lens. This image, however, will be inverted ; and its size, in pro- 

 portion to that of the object, will depend upon their respective distances from 

 the lens. If their distances be the same, their size will also be the same ; if 

 the object be distant, and the image near, the latter will be much the smaller; 

 and vice versa. 



535. There are two circumstances, however, which interfere with the per- 

 fection of an image thus formed by a convex lens. The one is, that, if the 

 lens constitute a large part of the sphere from which it is taken, the rays 

 which fall near its margin are not brought to a focus at the same point with 

 those which pass through its centre ; but at a point nearer the lens. This 

 difference, which must obviously interfere greatly with the distinctness of the 

 image, is termed Spherical Aberration ; it maybe corrected by the combi- 

 nation of two or more lenses, of which the curvatures are calculated to ba- 

 lance one another, in such a manner that all the rays shall be brought to the 

 same focus ; or by diminishing the aperture of the lens by means of a stop 

 or diaphragm, in such a manner that only the central part of it shall be used. 

 The latter of these methods is the one employed, where the diminution in 

 the amount of light transmitted is not attended with inconvenience. The 

 nearer the object is to the lens (and the greater, therefore, the angle of diverg- 

 ence of its rays), the greater will be the spherical aberration, and the more 

 must the aperture of the diaphragm be contracted in order to counteract*!!. 

 The other circumstance that interferes with the distinctness of the image, is 

 the unequal refrangibility of the differently-coloured rays, which together 



