3 
the two planets before me for several hours, so to speak, side 
by side in the field of the telescope at the same time, thus 
affording me a most perfect opportunity for making a com- 
parison of their relative brightness. It is difficult to convey 
in words an exact impression of the difference in the bright- 
ness of such objects, but I may attempt to do so by stating 
that Venus looked like clean silver while Mercury looked 
like lead or zinc. Were I to indicate my impressions by 
way of number I would say that Venus was fully twice as 
bright as Mercury. So remarkable an inferiority in the 
brightness of Mercury, notwithstanding his much greater 
nearness to the sun, appears to me to indicate the existence 
of some very special and peculiar condition of his surface 
in respect to his capability of reflecting light, a condition 
that may be due to the nature of his envelope, if such 
exist, or of that of his surface, by which the fervid light of 
the sun’s rays falling on him are in a great measure quenched 
or absorbed so as to leave but a small residue to be reflected 
from his surface. If this be so, it appears to me to be 
reasonable to suppose that the absorption of so much light 
must result in a vast increase in the heat of the surface of 
Mercury beyond what would have been the case had Mer- 
cury possessed the same surface conditions as Venus. 
Whether in the progress of spectroscopic investigation we 
shall ever be enabled to detect some evidence of metallic or 
other vapours or gases clinging to or closely enveloping the 
surface of Mercury that might in some respect account for 
so remarkable an absorption of the sun’s light, we must be 
content to await the acquirement of such evidence if it ever 
be forthcoming. It appears to me, however, to be well to 
raise such a question, so that our astronomical spectroscopists 
may be on the outlook for some evidence of the cause of so 
very remarkable a defective condition in the light-reflecting 
power of Mercury to which I have thus endeavoured to 
direct attention. 
