338 L. Page — A Century's Progress in Physics. 



one fifty-thousandth part of the radius of the atom. 

 Since numbers so small convey little meaning, consider 

 the following illustration, due, in part, to Kelvin. 

 Imagine a single drop of water to be magnified until it is 

 as large as the earth. The individual atoms would then 

 have the size of baseballs. Now magnify one of these 

 atoms until it is comparable in size with St. Peter's 

 cathedral at Rome. The electrons within the atom would 

 appear as a few grains of sand scattered about the nave. 

 This separation between the constituent electrons of the 

 atom, — so great in comparison with their dimensions, — 

 explains how alpha particles can be shot by the billion 

 through thin-walled glass tubing without leaving any 

 holes behind or impairing in the slightest degree the high 

 vacuum within the tube. The much smaller high-speed 

 beta particles pass through an average of ten thousand 

 atoms without even coming near enough to one of the 

 component electrons to detach it and form an ion. 



Michelson-Morley Experiment. — In 1881 Michelson 

 (22, 120, 1881) conceived an ingenious and bold method 

 of measuring the orbital motion of the earth through the 

 luminiferous ether. As the experiment was one involv- 

 ing considerable expense, Bell, the inventor of the tele- 

 phone receiver, was appealed to successfully for the 

 funds necessary to carry it through. Michelson's 

 experimental plan was as follows : A beam of light 

 traveling in the direction of the earth's motion strikes 

 an unsilvered mirror m at an angle of 45°. Part of the 

 light passes through, the rest being reflected at right 

 angles to its original direction. Each ray is returned by 

 a mirror at a distance I from m. On meeting again, the 

 ray whose path has been at right angles to the direction 

 of the earth's motion passes on through the mirror, while 

 the other ray is reflected so as to bring the two in line 

 and form interference fringes. Now consider the effect 

 of the earth's motion on the paths of the two rays. In 

 ^.g. 4 the earth is supposed to be moving to the right. 

 The unsilvered mirror m bifurcates a beam of light com- 

 ing from a source a. By the time the ray reflected from 

 m has traveled to the mirror b and back, m will have 

 moved forward to m' ; a distance 2 pi, where the small 

 quantity p is the ratio of the earth's velocity to the 

 velocity of light. Hence the length of the path traversed 

 by this ray is approximately 



