LESSONS IN PHYSICS. 



88 



adjusted a distinct image of objects in front of it may be seen 

 upon the screen by looking through the opening in the oppo- 

 site side of the box. When the image is properly focused the 

 photographer replaces the screen by a sensitized plate, and the 

 chemical powers of the sun's rays impress a true picture of the 

 object on this plate. (See Fig. 19.) 



The human eye is a camera which differs from the common 

 form of that instrument only in its wonderful perfection. 



A prism is a triangular transparent body that so refracts the 

 white light of the sun as to separate it into a band of colored 

 light called the solar spectrum. The solar spectrum is made up 

 of seven colors red, yellow, orange, blue, indigo, and violet, and 

 depends upon the fact that the waves of ether which give rise 



to the different colors are bent at different angles by refraction 

 through the prism. This is explained by the fact that the vibra- 

 tions which give rise to red light are longer and less rapid than 

 those which give rise to the yellow light, and are less refrangible; 

 that is, are bent less than those which give rise to yellow light, 

 and so on through the spectrum the waves which give rise to 

 violet light being shorter, more rapid and more refrangible than 

 those which give rise to any other color. Experiments show that 

 there are waves outside those which give rise to the red light, 

 which are less refrangible than the red waves, and which we may 

 recognize as heat waves; and there are other waves, which are 

 more rapid, shorter and more refrangible than the violet rays, 

 which we may recognize by their chemical effects. 



To the naked eye the colors of the spectrum seem continuous, 

 but when examined through a telescope the spectrum is found to 



