262 LIFE: ITS NATURE AND ORIGIN 



lieving. Tests indicate that insects have a visual wave-length 

 range different from ours; and human beings too have a relativity 

 in sensitiveness to light, for example, color blindness or Daltonism, 

 so called after the great chemist, who discovered it in himself. 

 Buildings are erected on the sufficiently close assumption that 

 plumb-lines are parallel, although we know that plumb-lines in 

 Istanbul are about at right angles to those in New York. 



There has been much controversy and probably also much 

 fraud in connection with "dowsing," the art of locating under- 

 ground streams of water with a "divining rod"; but so eminent a 

 scientist as Sir J. J. Thomson reports in his autobiography, 

 "Recollections and Reflections" (1938), that there is no doubt 

 about the reality of the phenomenon, even though at present we 

 have no explanation for it. Possibly it might be associated with 

 the ability of some persons to respond to radioactive, electric, or 

 other influences developed by flowing water (streaming potential). 

 Electroencephalograms and electrocardiographs recording elec- 

 trical impulses developed in our bodies, are matters of routine 

 determination in many hospitals. Homing pigeons have been ex- 

 tensively used for years, but we have no idea as to how they manage 

 to reach their home lofts from great distances. 



The results which the physicist deduces by mathematical logic 

 can never rise above the level of the accuracy of his data or assump- 

 tions — and there are limits to the accuracy of practical measure- 

 ments. The plea of Portia for Shylock's death "if the scale do 

 turn but in the estimation of a hair," is good drama, but bad 

 law; for the law in an analogous case would allow a reasonable 

 tolerance in weight. Engineers, however, generally insist that 

 "reasonable tolerance" be expressed in specifications, which may, 

 for example, call for rods having a length of five inches "plus or 

 minus one thousandth." With ball-bearings, turbines, etc., closer 

 tolerances are called for; and photo-cell devices and diamond- 

 tipped calipers are used to avoid errors that might follow wear 

 on less durable material. 



Heisenberg's Principle of Indeterminism 1 



When we attempt to measure such basic things as the velocity of 

 an electron and its location, we find that our very means of 

 measurement influence the result. This has led physicists to accept 

 Heisenberg's principle of indeterminism or uncertainty. In 

 common language this principle states that there is a limit to 



