CHAP. XVII.] PHENOMENA OF VISION. 45 



of the lens. It then emerges from the lens, and, in thus passing 

 from a dense to a rare medium, it is bent from the perpendicular 

 to the point of emergence. It can thus be shown that the com- 

 ponent rays of a pencil, diverging from a point, will be bent 

 towards the central ray of the pencil, or that which falls perpen- 

 dicular to the convex surface, and be brought to a focus in the 

 line of the central ray. And the several pencils of rays, proceeding 

 from different points of an object, cross one another in traversing the 

 lens, so that the foci to which they are respectively brought beyond 

 the lens are in positions the reverse of those from which they set 

 out, and the entire image is a reverse of the object. 



from the perpendicular to the point of emergence. The effect of this is to cause 

 a convergence of all the emerging rays towards the central ray ; at this point of 

 convergence or focus an image of the luminous point from which the rays 

 originally passed is formed. 



8. This point of convergence, or focus, varies as to its distance from the 

 surface of emergence, according to the refracting power of the lens, the amount 

 of curvature of its surfaces, and the distance of the luminous body. Its 

 distance constitutes the focal length or distance of a lens. 



9. In consequence of the unequal refraction of rays passing through a convex 

 lens, the focus of convergence of the central rays is more distant from the sur- 

 face of the lens than that of the peripheral rays. Hence the image formed at the 

 focus of the lens is in some degree indistinct at its edges. The imperfection is 

 due to what is technically called spherical aberration ; and it can be counteracted 

 only by intercepting the passage of the circumferential rays, or by employing 

 such a combination of lenses as will establish a just proportion between the 

 refraction of the central and peripheral rays. This aberration is liable to occur 

 in all forms of lenses, whether convex or concave. 



10. White light is compound, and may be analysed by passing a beam of sun- 

 light through a prism. The solar spectrum thus formed on a surface opposite the 

 prism is composed of bands of different colours insensibly passing into each other, 

 which are, beginning from above, violet , indigo, blue, green, yellow, orange, and red. 

 Of these different coloured rays, that which is most bent from the original direc- 

 tion of the solar beam is the violet, and the red is the least refracted. It is owing, 

 therefore, to this unequal refrangibility of the different kinds of simple light, 

 that we are enabled to decompose white light by its transmission through a 

 prism. 



11. If the different coloured rays which have emerged from the prism be 

 allowed to traverse a second similar prism, held in a reversed position, they 

 will, on their emergence, unite to form white light again. 



12. The different refrangibility of the rays of simple light is another source of 

 indistinctness in images formed by the transmission of light through lenses with 

 curved surfaces. The images are fringed by prismatic colours. It is called tech- 

 nically chromatic aberration; and may be corrected by means analogous to those 

 adopted for correcting spherical aberration. It is obviously of great importance 

 in all optical instruments for aiding or increasing the powers of vision, that they 

 should as niucL as possible be free from these sources of imperfection. 



