1885.] on how Thought p-esents itself in Nature. 179 



depends ou the periodic time of the waves that occur outside us, that 

 the quality of the tone depends on the form of the wave, and so on. 

 The character also of the imdulation in different materials has been 

 ascertained. For instance, in air it consists at each point of a rapid 

 succession of little winds blowing alternately forwards and back- 

 wards. The length of the waves in various media, the rapidity of 

 their advance, and a multitude of other particulars about them, have 

 been carefully measured. It has thus been ascertained that in air the 

 longest waves we can hear are about twelve or fourteen metres from 

 each point of compression to the next, the shortest about a centi- 

 metre ; that the length of the air waves produced by the middle E of 

 the piano is about one metre, and so on. 



Light is Motion. 



Another and a most important advance in our knowledge of 

 nature was made when discovery after discovery confirmed the truth 

 of the undulatory theory of light and radiant heat, and justified us in 

 accepting it. These and discoveries in molecular physics certify to 

 us that we see objects because periodic motions in their molecules 

 generate an undulation around that pulsates in harmony with them. 

 A very small portion of this great undulation obtains access to our 

 eyes through the pupils, and produces a change in some substance or 

 substances of the black pigment that lies under the retina at the 

 back of the eye. What this change is we do not yet fully know, but 

 it is probably a fugitive change of the nature of the more permanent 

 chemical change which occurs in photography. Of whatever kind it 

 is, the altered pigment gains for the time the power, through an 

 intricate apparatus of rods, cones, and bulbs, of exciting the optic 

 nerve to act upon the brain. However, we are not just at present 

 engaged in following up the series of events through our organs of 

 sense to the brain and mind, but are only inquiring what part of the 

 series takes place in the outer world. This part is simply motions : 

 anything else that is in the series of events known to us comes in at 

 some subsequent stage. 



What a Scientific Explanation is. 

 In order to keep our conceptions clear, it will be convenient to 

 take a general survey here of what would constitute a complete 

 explanation of a phenomenon from the scientific naturalist's point of 

 view. We may select any phenomenon — for example the green colour 

 we see when we look at the foliage of a particular plant, let us 

 suppose of a geranium. The perception of green in certain situa- 

 tions, which is, so long as we are looking at the plant, a part of the 

 great complex thought which we call our mind, is the known element 

 of this inquiry — equally known before the investigation as after it. 

 It may be called the a or known part of our problem ; and, adoptinc^ 

 the practice of mathematicians, we will assign the last letters of the 

 alphabet z, y, x, and lo to those unknown parts which the scientific 



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