32 MR. J. SMITH ON THE ORIGIN OF COLOUR 



Is it not in this case the interval between the vibration 

 of one wave and another I It is more. It is the sensation 

 of no-light. The interval between one wave and another 

 cannot be seen or appreciated ; for there is no sensation, 

 no negative, until the ray is bent from its original direc- 

 tion ; and if it is then seen, in its intervals of pulsation, 

 against a basis of a negative character, or against a plane 

 reflecting little or no light, the ray is changed in its cha- 

 racter. What is called white light may be changed to any 

 colour. But is not this the existing theory in different 

 words ? A moment's reflection will show that it is very 

 different. The present theory admits only positive terms. 

 What I am explaining demands two independent and co- 

 ordinate sensations in the production of colour. 



As every, even the most transparent, body casts a shadow, 

 so there is a probability that even the seemingly most 

 opaque body may, notwithstanding, transmit a certain por- 

 tion of light. There is, to all appearance, as great a variety 

 of shadow as there is of what is usually called light ; and 

 from this point of view shadow may be considered as a 

 positive principle, not from the darkness but from the light 

 which is in it. After a series of experiments we soon 

 find that it is as difficult to discover the value of a shadow 

 as of a ray of light. They have both eluded the most pro- 

 found researches. We are thus forced to the necessity of 

 vievnng shadow as a negative principle, but of different 

 values. In fact we must view it in many cases as a phase 

 of light, although, abstractly, it be perfectly inert. It is 

 this difficulty of not knowing what is light and what is not 

 light that causes such difficulties in our researches into 

 light.* An instance of this is to be found in Newton's 



* Dr. Keid sees no propriety at all in the term passive -power : " It is a 

 powerless power, and a contradiction in terms." 



Sir William Hamilton says : " This was one of the most celebrated dis- 

 tinctions in philosophy. . . . Power, therefore, is a word we may use both 

 in an active and passive signification, and in psychology we may apply it 



