Dr Brewster m a New Rejlecting Telescope. 
We believe it will be admitted by every person who has had 
much experience in the use of reflecting telescopes, that the 
Newtonian form is decidedly the best. Its construction is so 
simple, and it is so little liable to derangement from accidental 
causes, that, for popular use, on a small scale, it is superior to 
the Gregorian one, while, for instruments of great size, it is the 
only form that is practicable. But even when we consider it in 
a scientific point of view, it has the advantage of the Gregorian 
form. It is more easy to give a perfect figure to an uniform 
circular piece of metal than to a perforated disc ; — the spherical 
aberration is less, in so far as it is not increased by a second 
spherical mirror ; — ^and the quantity of light reflected from the 
oblique small speculum is decidedly greater than when it is re- 
flected at a vertical incidence. 
If we could dispense with the use of the small specula in te- 
lescopes of moderate length, by inclining the great speculum, 
and using an oblique, and consequently a distorted reflexion, as 
proposed first by Le Maire, we should consider the Newtonian 
telescope’^as perfect ; and on a large scale, or when the instru- 
ment exceeds twenty feet, it has undoubtedly this character, as 
nothing can be more simple than to magnify by a single eye- 
glass the image formed by a single speculum. 
As the front view is quite impracticable, and indeed has ne- 
ver been attempted in instruments of a small size, it becomes of 
great consequence, in a practical point of view, to remove as 
much as possible the evils which arise from the use of a small 
speculum. These evils may be thus enumerated. 
1. If we suppose the light reflected by the large speculum, 
or the light incident upon the small one, to amount to 10,000 
rays, then the light reflected by a well polished plane speculum 
will not exceed two-thirds of this, or 6666 rays, at a vertical in- 
cidence, and probably not above 6900 at an incidence of 45°^; 
consequently S200 rays out of 10,000 are lost by the use of the 
plane speculum. 
% Besides this great loss of light, we have to encounter all 
the errors of a second reflexion, arising from imperfections of 
aperture of 7| inches, the inclination of the incident and reflected measures is about 
eleven degrees. 
* The law of variation is yet undetermined. 
VOL. vn. NO. 14. OCT. 1822. 
y 
