50 Dr. Herschel on the Nature and Construction 
its natural changes, will explain such belts. Our spot in the 
sun may be accounted for on the same principles. The earth 
is surrounded by an atmosphere, composed of various elastic 
fluids. The sun also has its atmosphere, and if some of the 
fluids which enter into its composition should be of a shining 
brilliancy, in the manner that will be explained hereafter, 
while others are merely transparent, any temporary cause 
which may remove the lucid fluid will permit us to see the 
body of the sun through the transparent ones. If an observer 
were placed on the moon, he would see the solid body of our 
earth only in those places where the transparent fluids of our 
atmosphere would permit him. In others, the opaque vapours 
would reflect the light of the sun, without permitting his view 
to penetrate to the surface of our globe. He would probably 
also find that our planet had occasionally some shining fluids 
in its atmosphere; as, not unlikely, some of our northern lights 
might not escape his notice. If they happened in the unen- 
lightened part of the earth, and were seen by him in his long 
dark night. Nay, we have pretty good reason to believe, that 
probably all the planets emit light in some degree ; for the 
illumination which remains on the moon in a total eclipse can- 
not be entirely ascribed to the light which may reach it by 
the refraction of the earth's atmosphere. For instance in the 
eclipse of the moon, which happened October 22, 1750, the 
rays of the sun refracted by the atmosphere of the earth to- 
wards the moon, admitting the mean horizontal refraction to 
be 30' 50", 8, would meet in a focus above 189 thousand miles 
beyond the moon ; so that consequently there could be no il- 
lumination from rays refracted by our atmosphere. It is, 
however, not improbable, that about the polar regions of the 
