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University. On his return to America, after paying a visit to 
Parsonstown, where he had the advantage not only of making 
some observations with Lord Rosse’s large reflector, but also 
of seeing the method there pursued in grinding and polishing 
mirrors, stimulated by what he had seen, it was determined 
to build an observatory, and to construct an instrument to be 
devoted solely to celestial photography. The speculum used 
by Dr. Draper is fifteen and a-half inches in diameter, and 
twelve and a-half feet focal length ; but this was afterwards 
superseded by one of glass on Foucault’s principle. The great 
labour involved in a work of this character may be judged of 
by the fact that Dr. Draper ground and polished more than 
100 mirrors, varying in diameter from nineteen inches to a 
quarter of an inch ; but he appears at last to have secured a 
good instrument. The chief points to be noticed in this 
article are, that instead of driving the telescope in the usual 
way by means of a clock, the frame carrying the glass plate 
was made to move on the plan previously referred to. Instead 
of clockwork to effect this motion, an instrument called a 
“ clepsydra” was used. It has a weight and a piston rod, 
which fits into the cylinder filled with water, which is allowed 
to escape by means of a stopcock, and can he regulated with 
great exactness, so as to follow the object. The large number 
of 1,500 negatives are stated to have been taken at this 
observatory, some of which would bear magnifying twenty- 
five diameters (the paper says times , but I assume this to be 
an error, as a negative must be very bad if it will not bear 
more than five diameters, or twenty-five times.) As the 
average size of the negatives was liV inch, an increase of 
twenty-five diameters would give an image of the moon nearly 
three feet in diameter. I have not seen the prints from these 
negatives, and have never heard anything of the quality of 
the work produced by this telescope ; but it may he stated 
that Dr. Draper writes as if the negatives were of the best 
quality, and encourages others to follow his example. 
