USE F THE IMA01NA TION. 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 Brucke'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 
