SCIENTIFIC USE OF THE IMAGINATION. H9 



tering particles are small in comparison to the ethereal 

 waves, we have in the reflected light a greater propor- 

 tion of the smaller waves, and in the transmitted light 

 a greater proportion of the larger waves, than existed 

 in the original white light. The consequence, as re- 

 gards 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 

 go&ooth 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 ex- 

 ceeding in diameter the length of the smallest waves 

 of the visible spectrum.* 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 106 1 000 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 tur- 

 bid liquid was not to be distinguished from distilled 

 water, f 



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 



* Dallingor and Drysdale have recently measured cilia *Winnjth 

 of an inch in diameter. 1878. 



f- Like Dr. Burden Sanderson's ' pyrogen,' the particles of 

 mastic passed, without sensible hindrance, through filtering-pa- 

 per. By such filtering no freedom from suspended particles is 

 secured. The application of a condensed beam to the filtrate 

 renders this at once evident. 



