632 
POPULAE SCIENCE EEVIEW. 
slowly in twenty-four hours. The light which passes from the 
lamp travels to the mirror^ and is reflected by it (the rays having 
previously been made parallel by a lens) on the slowly- 
revolving cylinder. As the magnet moves slowly to the east 
or the west, so does the mirror, and so also does the spot of 
light upon the paper ; and the consequence is, that at the end 
of the twenty-four hours the paper, when removed from the 
cylinder, contains a series of curves which pass to the right 
or left of the centre, and thus indicate with the utmost exact- 
ness the movements which the needle has performed. In the 
case of the barometer a somewhat similar device has been 
employed; but in this case the mirror has been dispensed 
with. The light is allowed to traverse the barometer before 
passing on to the prepared paper, and as it can only pass 
at a point above the level of the mercury, the elevation or 
depression of this fluid is thus made to register itself. Thus, 
in the case of these several instruments does photography all 
the night long perform its useful office, and confer beneflts on 
mankind which only those who know the value of accurate 
scientiflc statistics can appreciate. We may mention, in 
passing, that the means of employing photography in the 
Observatory^^ are due entirely to the inventive mind of Mr. 
Brooke, a gentleman whose name our microscopic readers have 
often met with. Science has also profited by sun-pictures 
of the moon — a sketch of one of which appears in our pre- 
sent number — of great eclipses, of the dissected body, and — 
through the medium of the ophthalmoscope — of the interior of 
the human eye. 
It now only remains for us, in concluding this paper, to 
say a few words on the subject of two or three photographic 
novelties which can scarcely be classed under any of the above 
heads. First comes that of the hinograplij a form of picture 
which bids fair to become very popular. In the carte 
binograph one'’s double^'’ is represented ; in the same pic- 
ture may be seen two or even more representations of the same 
individual. If the result were arrived at by placing two or 
more pictures together, the efiect would not be curious ; but 
as it is, it is an interesting phenomenon. Fancy a portrait of 
a man having a pugilistic encounter with himself, or a woman 
pushing herself along in a perambulator, or a gentleman 
giving his double a light for his cigar ! Yet all these por- 
traits can be easily produced in the following manner. The 
camera having been adjusted, and the sitter placed at the 
right side of the held in the first position, a curtain of the 
blackest velvet is hung at a short distance from the lens in 
such a way as to cut off the remaining or left half of the field. 
The plate is now exposed for a few moments and covered. 
