December, 1913. 



KNOWLEDGE. 



443 



Polariscope phenomena are not often studied in 

 spectroscopy, and will not here be further developed. 



Refraction of Light. 



A wavy line is a fair representation of a 

 simple ray of light of a pure tint polarised in a 

 plane (see Figure 515). In this each particle has 

 moved on its upright line, and the wave has 

 advanced. A beam of common white light is 

 made up of myriads of such waves of different 

 lengths moving in different planes. It is the duty 

 of the spectroscope to sort and to spread these 

 mixed colours into a long strip in which the waves 

 of different lengths stand out distinctly in the order 

 of their speed or length. This sorting is effected by 



Figure 515. 



prisms or by a parallel set of very close, fine lines 

 ruled on metal or glass, and called a grating. We 

 will try to understand the action of a prism on a ray 

 of light. As long as a beam of light travels in a 



W 



Wf\TER. 



Figure 516. 



uniform medium, such as still air or still water, it 

 moves straight forward at a uniform speed. In 



angle. If the ray be a pure tint it is bent as in 

 Figure 516. If the pure ray pass through a prism 

 of glass it is bent both on entering and leaving, as 

 in Figure 517. 



Refrangibility of Light. 



A white beam of light may be made up of two 

 pure complementary colours. The best colours to 



Figure 518. 



use are green and red. If such a white beam passes 

 through a prism the two colours are differently bent, 

 and so separated as in Figure 518. 



The white beam of light from an " Osram " filament 

 lamp has a myriad of different wave-lengths. Those 

 the eye can see, range from about thirty- three 



Figure 519. 



thousand to sixty thousand to the inch. In this 

 case the prism separates them into a rainbow-tinted 

 streak with the red at one end and the violet at the 

 other, in the order shown in Figure 519. The red is 



Figure 517. 



n G E DB 

 Figure 520. 



INFRft RED 



passing from one medium to another a beam of least bent ; then in order come orange, yellow, green, 

 pure light alters speed, and if it falls obliquely on blue, indigo, and violet. All the colours shade into 

 the plane surface of separation it is bent at an one another and there are thousands of tints, each 



