LIGHT. 21 



it consists of infinitely small particles, emitted by luminous bodies ; 

 the other,— that it depends upon undulations, transmitted through 

 a highly elastic medium of extreme tenuity, called an ether, just 

 as sound is produced by the undulations of the atmosphere. 



Light travels in straight lines in every direction, with extreme 

 rapidity ; it occupies about eight minutes in coming from the sun to 

 the earth, which is at the rate of 200,000 miles in a second. 



When a ray of light falls on a plain surface, it may either be 

 absorbed, or reflected, or it may be transmitted through it. 



By reflection is meant that property which causes a ray of light, 

 striking upon a bright surface, to be thrown back, at an angle which 

 is always equal to the angle of incidence. 



Refraction of light, is where a ray in passing from one medium 

 into another of different density, is bent from its straight line. Where 

 the ray passes from a rarer to a denser medium, it is refracted 

 toivards a line perpendicular to the surface of the latter ; but when 

 it passes from a denser to a rarer medium, it is bent fi-oni a line 

 perpendicular to the surface of the denser substance. Different sub- 

 stances possess different degrees of refractive power ; generally 

 speaking, the densest substance refracts most ; the same is true also 

 of combustible substances. 



White light is compound, — that is, it is made up of different 

 coloured rays, as may be proved by admitting a ray of light through 

 a small aperture into a dark room, and 

 interposing a glass prism ; it will not only Fig. 6. 



be refracted from its straight course, but 

 will be decomposed into seven differently 

 coloured spaces, forming a figure termed 

 the solar spectrum. The upper part of 

 the spectrum is red ; the lower, violet ; 

 the intermediate portion, commencing with the violet, being indigo, 

 blue, green, yellow, and orange, all gradually shading off into 

 each other. These were termed by Newton the prismatic or 

 primary colours, from the impression that they were the elements of 

 white light. 



Brewster's opinion, which is rather the received one at present, is 

 that there are only three primary colours, viz., hhie, yellow, and red ; 

 and that when these are mixed in definite proportions, white light 

 will result ; but that when any of them is in excess, then the effect 

 of colour will be produced. The colours of natural objects are sup- 

 posed to result from the surfaces of these bodies absorbing certain 

 rays, and reflecting or transmitting others ; thus an object appears 

 red because it absorbs a portion of the yellow and the blue rays 

 composing the white light which falls upon it, while it reflects the 

 red rays. 



The greatest illuminating power of the spectrum is about its 



