

THE SENSE OF VISION 609 



be calculated. Water refracts the ray more than air, and glass more 

 than water. 



When a ray meets obliquely a piece of glass with parallel surfaces, 

 part of the light is reflected and part is refracted bent towards the 

 perpendicular to the surface. On again emerging at the other surface, 

 it is bent back again to its former direction, and therefore passes on 

 not in the same line as that in which it struck the glass, but in one 

 parallel to it. So, when light falls obliquely on the sides of a prism, 

 it is doubly bent, the amount of deflection depending upon the shape 

 and material of the prism. 



A similar effect is caused by lenses, both biconvex and biconcave. 

 A biconvex lens will bring to a focus all rays parallel to its principal 

 axis that is to. say, parallel to the line which passes through the centres 

 of curvature of its two surfaces. Such a point is termed the principal 

 iocus of the lens. Conversely, rays starting from the principal focus 

 will emerge from the lens in a parallel direction. 



Spherical Aberration. It has been stated that parallel rays falling 

 upon a biconvex lens meet at the focus. In practice, however, 

 this is not the case, as may be seen by trying to focus the sun's rays 

 on a piece of paper with a burning-glass. The image of the sun can 

 not be reduced to an absolute point, because the rays which 

 meet the circumference of the lens are more refracted than those 

 which fall nearer the middle of the lens. This is known as the spherical 

 -aberration of the lens. If the outer rays be cut off by interposing 

 a diaphragm, it is found that the image is made sharper. The iris 

 acts as a diaphragm and sharpens the image in the eye. If the 

 central part of the lens were made more refrangible than the outer 

 parts, then the rays nearer the centre would be more refracted than 

 the outer rays, and a similar result obtained. Such is the case with 

 the lens of the eye. 



Chromatic Aberration. It is found that, as is the case with a 

 prism, owing to the different degrees of refrangibility of the various 

 rays constituting white light, the latter is split up or dispersed into 

 its component colours in passing through a lens. The violet rays 

 are bent most, the red rays least. Therefore, if a scresn be inter- 

 posed before the focus, there will be an image with a violet centre 

 and a red edge; after the focus there will be an image with a red 

 centre and a violet margin. This is known as chromatic aberration. 

 Opticians remedy this by combining lenses of different powers of 

 dispersion, forming thereby the so-called compound achromatic 

 lens. 



In a system of lenses there exist six cardinal points. If the position 

 of these be known, then the direction of all rays through the system 

 can easily be traced. These six cardinal points are the first and second 

 focal, the first and second principal, and the first and second nodal 

 points: 



The first focal point is the point so placed that all rays from it, 

 after passing throiigh the system, emerge parallel to the axis of the- 



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