ANALOGY. 303 



1 i ) Be found to exist ; 



(2) May seem not to exist, but nevertheless may really 

 exist ; 



(3) May actually be non-existent. 



In the second case the failure is only apparent, and 

 arises from our obtuseness of perception, the smallness 

 of the phenomenon to be noticed, or the disguised cha- 

 racter in which it appears. I have already pointed out 

 that the analogy of sound and light seems to fail because 

 light does not bend round a corner, the fact being that it 

 does so bend in the phenomena of diffraction, which 

 present the effect, however, in such an unexpected and 

 minute form, that even Newton was misled, and turned 

 from the correct hypothesis of undulations which he had 

 partially entertained. 



In the third class of cases analogy fails us altogether, 

 and we expect that to exist which really does not exist. 

 Thus we fail to disc ,ver the phenomena of polarization in 

 sound travelling through the atmosphere, since air is not 

 capable of any appreciable transverse undulations. These 

 failures of analogy are of peculiar interest, because they 

 make the mind aware of its superior powers. There have 

 been many philosophers who said that we can conceive 

 nothing in the intellect which we have not previously 

 received through the senses. This is true in the sense 

 that we cannot image them to the mind in the concrete 

 form of a shape or a colour ; but we can speak of them and 

 reason concerning them ; in short, we often know them 

 in everything but a sensuous manner. Accurate investi- 

 gation shows that all material substances retard the 

 motion of bodies through them by substracting energy 

 by impact. By the law of continuity we can frame the 

 notion of a vacuous space in which there is no resistance 

 whatever, nor need we stop there ; for we have only to 

 proceed by analogy to the case where a medium should 



