SCIENTIFIC USE OF THE IMAGINATION 127 



precipitate, you may render the white light of noon as 

 ruby -colored as the sun, when seen through Liverpool 

 smoke, or upon Alpine horizons. I do not, however, 

 point to the gross smoke arising from coal as an illustra- 

 tion of the action of small particles, because such smoke 

 soon absorbs and destroys the waves of blue, instead of 

 sending them to the eyes of the observer. 



These multifarious facts, and numberless others which 

 cannot now be referred to, are explained by reference to 

 the single principle that, where the scattering particles are 

 small in comparison to the ethereal waves, we have in the 

 reflected light a greater proportion 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 regards sensation, is that in the one 

 case blue is predominant, and in the other orange or red. 

 Our best microscopes can readily reveal objects not more 

 than rfffonrth of an inch in diameter. This is less than the 

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

 scope 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 be- 

 cause they are smaller. Some months ago I placed in the 

 hands of our President a liquid containing Briicke's pre- 

 cipitate. The liquid was milky blue, and Mr. Huxley ap- 

 plied to it his highest microscopic power. He satisfied 

 me that had particles of even 100 i 000 th of an inch in diam- 

 eter existed in the liquid, they could not have escaped 



1 Dallinger and Drysdale have recently measured cilia TtfuV^ffth of an inch 

 in diameter. 1878. 



