SCIENTIFIC USE OF THE IMAGINATION. 119 



regards sensation, is that in the one case blue is pre- 

 dominant, and in the other orange or red. Our best 

 microscopes can readily reveal objects not more than 

 __i_tb of an inch in diameter. This is less than 

 the length of a wave of red light. Indeed a first-rate 

 microscope would enable us to discern objects not 

 exceeding in diameter the length of the smallest waves 

 of the visible spectrum. 1 By the microscope, therefore, 

 we can test our particles. If they be as large as the 

 light-waves they will infallibly be seen ; and if they be 

 not so seen, it is because they are smaller. Some 

 months ago I placed in the hands of our President a 

 liquid containing Briicke's precipitate. The liquid was 

 milky blue, and Mr. Huxley applied to it his highest 

 microscopic power. He satisfied me that had particles 

 of even 3 ^ O th of an inch in diameter existed in the 

 liquid, they could not have escaped detection. But no 

 particles were seen. Under the microscope the turbid 

 liquid was not to be distinguished from distilled water. 2 

 But we have it in our power to imitate, far more 

 closely than we have hitherto done, the natural con- 

 ditions of this problem. We can generate, in air, 

 artificial skies, and prove their perfect identity with 

 the natural one, as regards the exhibition of a number 

 of wholly unexpected phenomena. By a continuous 

 process of growth, moreover, we are able to connect sky- 

 matter, if I may use the term, with molecular matter 

 on the one side, and with molar matter, or matter in 

 sensible masses, on the other. In illustration of this, I 



1 Dallinger and Drysdale have recently measured cilia 20 o o6o th 

 of an inch in diameter. 1878. 



8 Like Dr. Burdon Sanderson's <pyrogen,' the particles of mastic 

 passed, without sensible hindrance, through filtering-paper. By 

 such filtering no freedom from suspended particles is secured. The 

 application of a condensed beam to the filtrate renders this at once 

 evident. 



