472 
POPULAR SCIENCE REVIEW. 
Mr. Dawes has been able to separate some exceedingly close 
double stars, and observe some very faint ones with such a 
telescope. The amateur will soon get accustomed to the 
difficulty of the various objects, and know what powers to 
apply, according to the fineness of the night or other circum- 
stances. Admiral Smyth — one of the most successful observers 
of the age — advises the amateur not to be afraid of his small 
instrumental means, but to look at everything ; and it is certain 
that with some costly apparatus much less is seen than with 
less pretentious affairs. 
For small instruments, the ordinary tripod stand, with slow 
motion in altitude and azimuth, is as convenient as any, 
where merely viewing the object is all that is required. For 
larger ones, where measurements of the positions and distances 
of double stars are to be obtained, the telescope is mounted 
equatorially — the support of the instrument, instead of being 
pointed to the zenith, being placed parallel with the axis of the 
earth — by which means, by merely shifting the telescope to 
the right, the star is always in the field of view without any 
vertical motion. To most large instruments strong clock- 
work is attached, by which this motion to the west is duly 
regulated, and the observer has only to keep his eye to the 
telescope, which of itself follows the star. Although in the 
last century and the time of Sir W. Herschel reflectors of 
from six to ten feet were greatly in use, yet of late years, 
in the case of small telescopes, it has been almost com- 
pletely superseded by the refractor, and on that account 
we have confined our observations to this most convenient 
description of optic tube. Unless extreme care is used with a 
reflector, the metal mirror is very liable to become tarnished, 
whilst dew or dust settling on a glass lens can easily be 
wiped off with a soft cloth. In France, however, small re- 
flectors are again coming into vogue, the mirror being formed 
of glass and coated with a thin film of silver ; and as the latter 
has a reflective power of 91 to 67, in comparison with 
speculum metal, it has a great advantage in this respect ; 
but, unfortunately, tarnishes very readily. As, however, the 
silver film is easily renewed, this is of the less consequence. 
We had the pleasure of inspecting several instruments of this 
description by the celebrated M. Foucault, at Secretan’s esta- 
blishment at Paris, some time since, which were beautifully 
fitted up. A reflector of about forty inches aperture has since 
been made by those gentlemen, and as it w'as with one of those 
glass mirrors that the companion of Sirius has been detected, 
it may give an idea of then merit. 
The largest refracting achromatic telescope we have seen, 
and which we believe to be at present the largest in the world. 
