METHOD OF DISCOVERING CAUSAL LAWS 185 



The following example 1 will prove instructive as showing that the 

 method of difference often requires to be supplemented, and can be supple 

 mented, by the experimental examination of positive instances ; in other words, 

 by the experimental application of the method of agreement to a number 

 of positive instances ; the object being to determine what precise factor in 

 the ^previously observed &quot;invariable antecedent&quot; is really the factor that is 

 relevant or essential to the effect. As early as the thirteenth century Roger 

 Bacon had inferred, by the method of agreement, that the passage of light 

 through transparent globes, prisms, crystals, etc., was probably connected 

 causally with the production of &quot; rainbow &quot; colours. Four centuries later, Sir 

 Isaac Newton commenced his investigation of the phenomenon by an appli 

 cation of the method of difference. &quot;A beam of the sun s light, admitted 

 through a small hole in an otherwise darkened room, produces on a screen a 

 circular image of the sun (negative instance). But on passing the beam 

 through a prism, the image becomes nearly five times as long as it is broad, 

 and is coloured from end to end by a succession of vivid tints (positive in 

 stance). Hence something in the glass is the cause of the colours [assuming 

 that no other factor was unconsciously introduced simultaneously with the 

 prism].&quot; But what was the &quot; something &quot;? Was it (a) the particular size of 

 the prism ? (b) the particular quality of the glass ? (c) the particular position 

 in which the prism was held ? (d) the particular temperature of the glass ? (e) 

 the substance itself (glass) of the prism ? No. It was none of all these ; for 

 Newton proceeded to vary all these, to eliminate them successively in a 

 series of experiments, in all of which he retained what he himself suspected 

 to be the essential factor, viz. the prismatic shape of the various trans 

 parent media which he employed. The instances were all positive instances, 

 and the &quot; prismatic shape &quot; (of the medium) was common to all of them. 

 &quot; He eliminated this [presumably in every instance] by placing on the 

 original prism a second one of exactly the same angle, but inverted, so that 

 together the two prisms formed a solid with parallel surfaces &quot; ; thus securing 

 in each case a negative instance, and making each experiment an application 

 of the method of difference. He next conceived the hypothesis that white 

 light is really composed of various primary rays which are subject to refrac 

 tion in unequal degrees from the red which is least, to the violet which is 

 most, refrangible. This hypothesis he verified by passing each colour of the 

 spectrum separately through a hole in the screen, and then through a second 

 prism : the latter refracting each of the rays in different degrees without 

 further decomposing any of them. 



the sugars as does the living yeast cell. . . . According to Buchner the fermentative 

 activity of yeast-cell juice is not due to the presence of living yeast cells, or to the 

 action of living yeast protoplasm, but it is caused by a soluble enzyme. [But other 

 investigators brought to light certain facts which] cannot be explained by the theory 

 that it is a soluble enzyme which brings about the alcoholic fermentation of sugar. 

 The remarkable discoveries of Fischer and Buchner . . . reconcile Liebig s and 

 Pasteur s theories. Although the action of zymase may be regarded as mechanical, 

 this enzyme cannot be produced by any other than living protoplasm.&quot; Encyclo 

 pedia Britannica, eleventh edit, vol. 10, p. 276. 



1 From BADEN-POWELL S History of Natural Philosophy, quoted by Dr. 

 MELLONE, op. cit., p. 301 ; cf. ibid., p. 298. 



