158 PROCTOR, ON LIGHT. 
comet is imperfectly transparent, and we do not know whether 
even the luminiferous ether itself allows the passage of light 
without some loss, but we know that glass is as much opaque 
compared with it as gold is when compared with glass; and 
from this we readily learn to believe that transparency and 
opacity are only comparative terms—that nothing transmits 
all the light, and nothing is entirely impervious to light; and 
this supposition is confirmed by experience, so far as I have 
been able to examine so-called opaque bodies in a way which 
gave reasonable prospect of satisfactory results. I will not 
trouble you with a detailed account of the experiments, but 
simply refer you to the table, and the objects to be seen 
under the microscopes : 
LIGHT TRANSMITTED. 
Through Gold leaf. . . . .is Green. 
a > “Sempered |...) 2” Brown, 
o »» Chemical films . Gray-violet. 
= oS powder. Red, purple, or blue. 
ss Silver leaf . . . . Gray-violet. 
% », Chemical films . Purple or brown. 
> Copper’, “c= se 2. Green. 
4 Anbimony <7 ess 5. Aataye 
st Arsenie™” 2° 204 ne = brown: 
. latinas 5/0 i s Maray, 
4 Palladian. § 3°05". "o 2 aoray 
i Rhodium’ «©. . . «.° *Brown*or' bine: 
of Charcoal 1 2 2 ee Gray, 
A Todine * oS MS + ed=brown: 
There are several objects on this list of which I have not 
got specimens; they will be found fully described, and their 
mode of preparation explained, in a paper by Faraday on 
“Gold in relation to Light,” which was published in the 
‘Philosophical Magazine’ a year or two since. 
The tempered gold and silver leaf are remarkable for their 
great transparency, compared with that which has been 
beaten since it was annealed. May we suppose that the 
greater mobility in the molecules which characterises the 
annealed metal facilitates the luminous undulations? Or 
that there is an increased distance between the molecules 
which allows of the undulations passing more freely between 
them ? 
The chemical film of silver has two colours—inky purple 
at one part, and brown at another; probably others of the 
