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are small in comparison to the size of the waves, we 

 have in the reflected light a greater proportion of the 

 smaller waves, and in the transmitted light a greater pro- 

 portion of the larger waves, than existed in the original 

 white light. The physiological consequence is that in the 

 one light blue is predominant, and in the other light orange 

 or red. And now let us push our inquiries forward. Our 

 best microscopes can readily reveal objects not more 

 than ^o(7ff f an mc ^ m 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 submit our particles to an experimental test. If 

 they are as large as the light-waves they will infallibly 

 be seen ; and if they are not seen it is because they are 

 smaller. 



I placed in the hands of our president a bottle con- 

 taining Briicke's particles in greater number and coarse- 

 ness than those examined by Briicke himself. The 

 liquid was a milky blue, and Mr. Huxley applied to it 

 his highest microscopic power. He satisfied me at the 

 time that had particles of even 1T5 o\joo 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 distin- 

 guished from distilled water. Briicke, I may say, also 

 found the particles to be of ultra microscopic magni- 

 tude. 



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

 than we have hitherto done the natural conditions of 

 this problem. We can generate in air, as many of you 

 know, artificial skies, and prove their perfect identity with 



