SIR ISAAC NEWTON 105 



their path. The buoyancy of the air will affect light masses 

 more than dense ones; if the air be removed all substances will 

 fall to earth with equal rapidity. Newton's great step was the 

 extension of the idea of gravitation to celestial objects; and 

 he claimed that all masses attract each other at all times, but 

 that distance influences the intensity of this attraction. 



By means of Newton's generalization astronomers have been 

 able to calculate the mutual attraction of the various planets 

 for each other and for the sun. The more closely celestial 

 bodies approach one another, the more they affect each 

 other's motions; with the help of Newton's law these irregu- 

 larities in the orbits have been explained. The matter has 

 been carried even further; from such perturbations of Saturn, 

 Leverrier was able to predict the existence of the then un- 

 known planet Neptune, and to tell owners of better tele- 

 scopes than he possessed where to look for it. 



Again, Newton was the first to explain the tides, and to 

 show how they were due to the action of the moon and the 

 sun upon the ocean, although the larger body, the sun, is so 

 much farther from the earth than is the moon that its attrac- 

 tion is less apparent. The moon appears to revolve around the 

 earth once in about twenty-four and one half hours, whereas 

 the average interval between the successive appearances 

 of the sun is twenty-four hours. The solar tide and lunar tide 

 therefore pursue unequal rates. On the occasions when they 

 come together we have the high or spring tides. When they 

 oppose each other we have the low or neap tides. 



More practical still were Newton's discoveries in optics. 

 Telescopes had been made by Galileo and others before 

 him, but the lenses employed were imperfect owing to the 

 difficulties in the manufacture of glass, and the light in pass- 

 ing through them showed queer colorations. The images 

 were blurred and surrounded by colored fringes. It is only 



