I50 SCIENCE PROGRESS 



law considered in detail, and is given to illustrate the complexity of laws. 

 Emphasis is laid upon the numerous other laws which may be involved in 

 its verification. To measure the extension, often very small, we may use 

 an optical lever involving the laws of reflection of hght or a level and micro- 

 meter screw. It may be urged that such experimental devices are merely 

 a matter of convenience. The truth of Hooke's law does not depend upon 

 the laws of reflection of Hght ; if there were any doubt about these laws, 

 we could adopt another method which does not involve them. Campbell 

 urges, however, that if we confine ourselves to the only direct method, involving 

 merely a millimetre scale, the scope of the law \vill be very much restricted 

 (owing to the smallness of the extension ?). Indirect methods are therefore 

 necessary involving other laws. " But there is one law common to them all, 

 namely that they aU give the same result." He therefore concludes that 

 " it seems impossible to accept the view that the laws involved in the use 

 of experimental devices are in no way essential to the laws which they are 

 employed to prove ; it seems to be indicated that a law, namely that different 

 methods of measurement agree, is involved in the use of the term ' extension.' " 

 He then considers the conception of force, defining it as the product of mass 

 into acceleration, in illustration of a similar complexity. This example 

 is so characteristic of the general nature of the book that a few comments 

 on it are desirable. We are suddenly plunged into the middle of a complex 

 phenomenon. It is impossible to speak rationally of extension and force 

 until these have been defined. To urge this in a three-page argument may 

 be appropriate enough in explaining the matter to " a person of average 

 intelligence, but wholly devoid of scientific knowledge — say, the minister in 

 charge of a Government department." What the Government minister 

 would make of the explanation we do not feel sure. He would probably 

 go away thinking that science is not as straightforward as he has been asked 

 to beUeve. \\Tien he meets \vith difficulties as to what force is, he is supposed 

 to be told that it is mass multiplied into acceleration ; but he is not told 

 either what mass or acceleration is, nor even how you can multiply them 

 together. No doubt these further explanations are supposed to be given in 

 retrograde sequence as the minister drives his informant more and more into 

 a comer. But the order seems to be curiously topsy-turvy in a book written 

 specially for professional physicists and not for the instruction (or mystifi- 

 cation) of departmental ministers. 



We have chosen this example because it illustrates a disorderUness which 

 it seems to us is characteristic of the whole book. We regret that space does 

 not permit us to demonstrate this statement in detail. 



There is also a tendency to push arguments up to a stage where they 

 become fanciful. Thus (p. 50), he argues that all laws are connected, though 

 in many cases the connection may be so sUght as to become inappreciable. 

 Thus Hooke's law involves the conception of force. But the force brought 

 into play may, in a particular case, arise from the incidence of radiation 

 (the " pressure of radiation"), and radiation is an electrodynamic phenomenon. 

 Hence there is a connection between Hooke's law and any law in which the 

 laws of electrodynamics are involved. And the author adds, " No doubt 

 this conclusion seems fanciful, and it is not pretended that it is of the slightest 

 importance in the practice of physical investigation." The general statement 

 as to the interconnection between phenomena may be conceded, but the 

 illustration seems a singularly unhappy one. The laws of electromagnetics 

 might change so much that there was no pressure of radiation at all, and 

 Hooke's law would not be in the slightest degree affected. What might 

 be affected would be the connection between the extension and (say) the 

 magnetic vector in the incident wave ; but this connection is, of course, not 

 Hooke's law. It may quite plausibly be regarded as one of the prime func- 

 tions of the scientific investigator to seek out the quantities whose inter- 



