CREATIVE SCIENCE 339 



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Rutherford's genius has been well characterized in this connexion by 

 one [C. G. Darwin] who knew him closely. He could throw aside as 

 irrelevant a stream of reports pouring in from all over the world about 

 new oddities to which fellow scientists called his attention, and yet 

 respond to one particular instance among them, raising a hue and cry 

 such as caused [his colleague and former student] Chadwick to dis- 

 cover the neutron. 



What factors determine the individual's setting of the TI? 



Knowledge and expectation. F.ven to recognize a discrepancy as 

 such, prior knowledge is required. One must know enough of the 

 theories and laws of the science concerned to anticipate what should 

 happen; and one must know enough of its experimental procedures to 

 estimate how closely that anticipation should be borne out. Some 

 more specific knowledge may also be required to recognize a phe- 

 nomenon in some deviation from the base line of expectation. Hille- 

 brand, no expert in spectroscopy, saw nothing in the spectrum of the 

 gas from clevite but a failure to correspond exactly to the spectrum of 

 nitrogen— and he set this failure down as an artifact of the experi- 

 ment. But Crookes, a highly experienced spectroscopist, no sooner 

 set eye to eyepiece than he saw that several of the spectral lines were 

 identical with some earlier attributed to a hypothetical solar element, 

 helium. Given his prior knowledge Crookes saw more than an arti- 

 fact, more even than a discrepancy. He saw a phenomenon: the ter- 

 restrial occurrence of helium. 



Whether or not we call Crookes' discovery accidental is primarily 

 a matter of definition. Certainly he had no anticipation that he was to 

 find helium. But, unlike Hillebrand, he was prepared to find some- 

 thing more than nitrogen: he thought argon might be present. Thus 

 he brought to the examination of the spectrum a different "set": he 

 was less interested in the resemblance of the spectrum to that of 

 nitrogen than in any perceptible differsnce between the two spectra. 

 For any such difference(s) the TI was set low and, a difference being 

 noted, Crookes' past experience at once made it a phenomenon. 



Pasteur writes : 



In the fields of obsei-vation, chance favors only the mind which is 

 prepared. 



Even better than the Crookes episode, Pasteur's own work illustrates 

 again and again the value of such preparation. Thus, for example, 



