$ 13] The Dialogue on the Two Chief Systems 165 



brightness to be nearer than the other. It is, for example, 

 evident that if, in the figure, E, E' represent two positions of 

 the earth in its path round the sun, and A, B two stars at 

 different distances, but nearly in the same direction, then 

 to the observer at E the star A appears to the left of B, 

 whereas six months afterwards, when the observer is at E', 

 A appears to the right of B. Such a motion of one star with 

 respect to another close to it would be much more easily 

 observed than an alteration of the same amount in the 

 distance of the star from some standard point such as the 

 pole. Salviati points out that accurate observations of 



E 



FIG. 57. The differential method of parallax. 



this kind had not been made, and that the telescope might 

 be of assistance for the purpose. This method, known as 

 the double-star or differential method of parallax, was in 

 fact the first to lead two centuries later to a successful 

 detection of the motion in question (chapter xin., 278). 



130. Entirely new ground is broken in the Dialogue 

 when Galilei's discoveries of the laws of motion of bodies 

 are applied to the problem of the earth's motion. His 

 great discovery, which threw an entirely new light on the 

 mechanics of the solar system, was substantially the law 

 afterwards given by Newton as the first of his three laws 

 of motion, in the form : Every body continues in its state of 

 rest or of uniform motion in a straight line, except in so far 

 as it is compelled by force appl'ei to it to change that state. 

 Putting aside for the present any discussion of force, a 

 conception first made really definite by Newton, and only 

 imperfectly grasped by Galilei, we may interpret this law 

 as meaning that a body has no more inherent tendency to 

 diminish its motion or to stop than it has to increase its 

 motion or to start, and that any alteration in either the 

 speed or the direction of a body's motion is to be explained 

 by the action on it of some other body, or at any rate by 



