602 



NATURE 



[Oct. 1 8, 1883 



be a clear and distinct one, that formed by our glass lens would 

 be a very bad one ; instead of the poles of the electric arc being 

 clearly and sharply defined, they would appear as if seen in a 

 hue, and would be surrounded by coloured fringes of light, and 

 not much could be made of thern. Why is this ? We find by 

 experiment that this attempt to imitate the action of the eye by 

 means of such a simple glass lens is an incorrect way of proceed- 

 ing, the eye possessing certain qualities which the simple glass 



lens does not. Although a lens seems to be a very simple 

 matter, its structure is really based upon some very complicated 

 considerations. If a section of it be Jaken it will be seen that 

 its surface is built up of sections of triangular pieces of glass, 

 these triangular pieces of glass being called prisms, and how 

 they deal with the light it is very important for us to know. If 

 in front of the beam of light issuing from the lantern a prism be 

 interposed, it will be found that whilst part of the light is re- 



Fig. 14. — Diagran explaining the formation of an achromatic lens, a, crown-glass prism ; r.. fl nt-gl iss prism of less angle, but giving the same amount 

 of colour ; c, the two prisms combined, giving a colourless yet deviated band of light at D". 



fleeted from its first surface another portion is refracted as it 

 is termed, that is, bent out of its original course by the prism. 

 Further, it not only suffers this deviation due to refraction, but it 

 undergoes also what is called dispersion. In ftct, where the 

 light falls on the screen an infinite number of different colours 

 are seen, these forming what is called a spectrum. This is one of 

 the reasons why such a glass lens as we have used will not per- 

 form the finer work of the eye ; the images of the poles are 



surrounded by a false glow, because it is difficult to give the lens 

 the proper curvature, and there is this power of dispersion which 

 breaks the compound white light up into a number of its different 

 elementary colours. It is this power of deviation which the lens 

 possesses which enables it to bend the rays differently according 

 to their different distances from its centre, and causes them to 

 form an image at what is termed the focus of the lens. The 

 rays of light passing through the outer part of the lensjundergo 



Fig. 15. —Teles-ope. A, ol\ject-glass, giving an image at b; c, lens for magnifying image b. 



more deviation in order that they may be brought to a focu; 

 at the same point as the other rays. ' Now prisms which are 

 made of different material, although the/ .be of the same 

 size and of the same ang'e, produce .different deviati >ns 

 and different dispersions of the light which falls upon them. 

 This fact has been taken advantage of in the c instruction of 

 lense . Let us take an Illustration of the way in which 

 this has been done. Imagine glass which gives a high dis- 



persion and but slight deviation, set to work against glass 

 giving great deviation with but little dispersion. It is obvious 

 that it is quite possible by a combination of that character to 

 keep the deviation and get rid of the dispersion, or to keep the 

 dispersion and get rid of the deviation, as may be desired. By 

 doing this an artificial eye of great excellence may be made. 

 Sup] ose two different kinds of glass so combined as to form a 

 prism, which should give a perfectly white image. Then the 



