280 



SCIENCE 



[N. S. Vol. XLIV. No. 1130 



tional forces, but the latter come into action 

 with velocity, while the former come into play 

 with acceleration." On page 152 he states 

 that " both forces and kinetic reaction must 

 be the same type of magnitude." 



These statements, together with others, seem 

 to indicate that the author considers kinetic 

 reaction as something real and of the nature 

 of a force. In fact it is a force, although the 

 author on page 150 states that kinetic reaction 

 can not be called a force because we have re- 

 stricted the latter term to the action of one 

 material body upon another. Call it what we 

 will, to the reviewer it seems to be nothing 

 more nor less than a backward pull of the ether 

 on a body as the body moves through the ether 

 with accelerated motion. In fact, the author 

 seems to say that the inertia of a body is due 

 to the force with which the ether is pulling 

 back on a body when the body is being accel- 

 erated. 



Assuming that the author's conception of 

 kinetic reaction is here correctly given, the re- 

 viewer is inclined to believe that several ques- 

 tions will at once present themselves to the 

 readers of his book. 



Why is it that the ether acts on a body only 

 when it is being accelerated and not when the 

 body is moving with constant velocity? 



If kinetic reaction is the action of the ether 

 on a particle, and if it is the same kind of a 

 quantity as force (is a force in fact), and if the 

 resultant force F acting on a particle and the 

 kinetic reaction q are always equal in magni- 

 tude but opposite in direction (both equal to 

 ma in magnitude), why is the body not in 

 equilibrium? The author recognizes this diffi- 

 culty in a footnote (page 153) by stating in 

 effect that we must not call kinetic reaction a 

 force, for if we do then the vector sum of all 

 the forces acting on a particle will always 

 equal zero without this particle necessarily 

 being in equilibrium, a state of affairs which 

 is not consistent with the condition of equilib- 

 rium of a particle. Refusing to call kinetic 

 reaction a force, however, in order to keep out 

 of trouble simply dodges the question and does 

 not answer it. 



The reviewer does not wish to say that the 



author is wrong in his conception. All he 

 wishes to say is that he entirely fails to appre- 

 ciate the author's point of view. 



There is considerable difference between the 

 author's action principle and D'Alembert's 

 principle. Let there be a number of forces act- 

 ing on a particle, then the resultant force (an 

 ideal force) equals ma, or B = ma. This ideal 

 force may be called the effective force. D'Alem- 

 bert's principle then says that a system of forces 

 acting on a particle together with the reversed 

 effective force will form a system of forces in 

 equilibrium. It should be remembered that 

 this reversed effective force is an ideal force 

 and not a real force. Now in the author's 

 action principle the kinetic reaction is a real 

 force (or action as the author prefers to call 

 it) and is due to the action of the ether on a 

 particle. 



The author's action principle (even if sound) 

 involves a number of conceptions which must 

 be understood in order to understand the prin- 

 ciple itself, and it seems that such a principle 

 ought to follow rather than precede an ele- 

 mentary treatment of mechanics. 



E. W. Rettger 

 Ithaca, N. Y. 



SPECIAL ARTICLES 



EXPERIMENTAL ABLATION OF THE HYPOPH- 

 YSIS IN THE FROG EMBRYO 



In the following preliminary paper the effect 

 of the extirpation of the epithelial portion of 

 the hypophysis upon the subsequent growth 

 and development of tadpoles is summarized. 

 The work was first attempted in 1914, Diemy- 

 ctylus torosus being used, repeated in 1915 

 upon Bana pipiens, and again repeated in 1916 

 upon Bana boylei. In this paper the results 

 obtained with B. boylei are reported. 



The operation was most successfully carried 

 out upon approximately 3 mm. larva?, at which 

 time the tail-bud is forming and the stoma- 

 deum can be detected. At that stage the 

 epithelial hypophysial invagination can be ac- 

 curately determined from the pit that it forms, 

 or from its location between the protuberance 

 of the forebrain and the stomadeum, and can be 

 removed without injury to the adjacent brain. 



