180 ELEMENTARY GENERAL SCIENCE CHAP. 



the refraction is unaccompanied by dispersion. The second 

 prism undoes the dispersive work of the first (Fig. 92). 



Or if, after the first prism, a double convex lens were arranged 

 so that the decomposed light falls upon it, it will be found that 

 the dispersed light is recombined and a white image is formed 

 at the principal focus of the lens. 



The Colour Disc. This is an arrangement of Newton's for 

 demonstrating that white light can be made by combining the 

 various colours of the spectrum. 



EXPT. 175. Upon a round piece of card paint sectors of 

 the different colours contained in the spectrum, arranging the 

 areas of the coloured sectors as nearly as possible in the pro- 

 portion in which they occur in the spectrum. 



Place the card upon a whirling table (Fig. 93) or upon a 

 top, and rotate it rapidly, when it will be found that light 

 from the card gives rise to the sensation of white or gray. 



CHIEF POINTS OF CHAPTER XI. 



Light, like every kind of radiation, is a form of energy. Hence 

 it is a process of transference of energy by ether-waves. These 

 ether-waves which affect the retina are known as light. 



Rectilinear Propagation of Light. Light travels in straight lines 

 when propagated through any one medium, but often has its 

 direction changed when passing from one medium to another (see 

 Refraction). 



Reflection. Light in being reflected from suitable surfaces, obeys 

 the following laws : 



(1) The reflected ray lies in the same plane as the incident ray 

 and the normal. 



(2) The angle of reflection is equal to the angle of incidence. 

 Reflection from Spherical Mirrors. The centre of the sphere of 



which the mirror is a part is called the centre of curvature. The 

 distance from this point to the reflecting surface is called the radius 

 of curvature. The point to which all parallel rays converge after 

 reflection is called the principal focus. The distance of the principal 

 focus from the pole of the mirror is called the focal length of the 

 mirror and is equal to one half of its radius of curvature. 



Refraction of Light. A ray of light passing from a less dense to a 

 more dense medium is bent towards the perpendicular to the sepa- 

 rating surface and conversely. The laws of refraction can be stated 

 thus : 



(1) The incident ray, the normal, and the refracted ray are all in 

 the same plane. The incident and refracted rays are on opposite 

 sides of the normal. 



