338 GENERAL SCIENCE 



search for some unknown cause of the variation. Neptune, 

 one of the eight planets of the family of the sun to which 

 our earth belongs, was located (discovered) by astronomical 

 calculations. The telescope verified its existence when 

 the instrument was directed toward that part of the heavens 

 where the hitherto unknown planet had been mathemati- 

 cally located. 



SUMMARY 



A shadow is the space from which light is more or less completely 

 shut off by an opaque body. It extends outward from the body on 

 the side opposite the illumination. The forms seen on screens and 

 walls, and commonly called shadows, are cross-sections of the shadow 

 region. 



An eclipse is a phenomenon where an illuminated body darkens or 

 completely disappears by reason of its entering the shadow of another 

 body. An eclipse of the moon occurs when by reason of its revolution 

 it comes into the shadow of the earth. Astronomers can calculate the 

 times of occurrence of eclipses with the greatest accuracy. 



The outer portion of the sun is an atmosphere of gases and vapors 

 thousands of miles deep, and this atmosphere is so highly heated as to 

 be luminous. From this "solar atmosphere" light and heat come to 

 the earth. In it are found in gaseous form nearly all known chemical 

 elements. 



The existence of non-luminous bodies far out in space, and wholly 

 invisible because they are non-luminous, is made known to astronomers 

 by the effect they have upon the motions of bodies which can be seen. 

 Where the actual position or path of motion of a luminous body differs 

 materially from its calculated place, astronomers feel justified in 

 asserting the existence of one or more invisible bodies whose pull of 

 gravitation occasions the change in position or in motion. 



IMAGES BY REFLECTION IN PLANE MIRRORS 



For a satisfactory understanding of the common phenom- 

 ena of image formation in plane mirrors such as looking- 

 glasses, a knowledge of the theory of light as taught in Physics 

 is required. Whatever light may be (page 344), it is very 



