USM ob' THE IMAGINATION. 433 



for fifteen or twenty minutes under the continual operation 

 of the light. During these fifteen or twenty minutes the 

 particles constantly grow larger, without ever exceeding 

 the size requisite to the production of the celestial blue. 

 Now when two vessels are placed before us, each containing 

 sky-matter,, it is possible to state with great distinctness 

 which vessel contains the largest particles. The eye is very 

 sensitive to differences of light, when, as in our experi- 

 ments, it is placed in comparative darkness, and the wave- 

 motion thrown against the retina is small. The larger 

 particles declare themselves by the greater whiteness of 

 their scattered light. Call now to mind the observation, 

 or effort at observation, made by our president, when he 

 failed to distinguish the particles of mastic in Briicke's 

 medium, and when you have done this, please follow me. 

 A beam of light is permitted to act upon a certain vapor. 

 In two minutes the azure appears, but at the end of fifteen 

 minutes it has not ceased to be azure. After fifteen min- 

 utes its color, and some other phenomena, pronounce it to 

 be a blue of distinctly smaller particles than those sought 

 for in vain by Mr. Huxley. These particles, as already 

 stated, must have been less than a hundred thousandth 

 of an inch in diameter. And now I want you to consider 

 the following question: Here are particles which have been 

 growing continually for fifteen minutes, and at the end of 

 that time are demonstrably smaller than those which defied 

 the microscope of Mr. Huxley. What must have been the 

 size of these particles at the beginning of their growth? 

 What notion can you form of the magnitude of such par- 

 ticles? The distances of stellar space give us simply a 

 bewildering sense of vastness, without leaving any distinct 

 impression on the mind; and the magnitudes with which 

 we have here to do bewilder us equally in the opposite 

 direction. We are dealing with infinitesimals, compared 

 with which the test objects of the microscope are literally 

 immense. 



Small in mass, the vastness in point of number of the 

 particles of our sky may be inferred from the continuity of 

 its light. It is not in broken patches, nor at scattered 

 points, that the heavenly azure is revealed. To the 

 observer on the summit of Mont Blanc, the blue is as uni- 

 form and coherent as if it formed the surface of the most 

 close grained solid. A marble dome would not exhibit a 



