D. Trowbridge on the Invisibility of Nehulous Matter. 211 
certain permanent conditions, and we are under the necessity of 
looking to physical causes inherent in the body itself for an ex- 
planation of some few observations which appear irreconcilable 
with the theory of reflected solar light.”* “The molecular con- 
ditions of the head or nucleus, so seldom possessing a definite 
outline, as well as the tail of the comet, is rendered so much the 
more mysterious from the fact that it causes no refraction.” * 
have collected these facts together to show that reflected 
solar light cannot completely explain, at present, all the phe- 
nomena of the light of comets. Besides the above observations, 
it may be added that the visibility of comets in the day time, 
and even when near the sun, also indicates a light-generating 
process in the comet; for otherwise we must suppose them ca- 
that nebulous vapor is necessarily too diffuse, 
. 
render nebulous matter visible 
