Dr Brewster cm a New Rejiecting Telescope. 
great value in the magnificent instruments of Sir William Her- 
schel. In this case, the small speculum was laid aside ; and 
the observer, placed with his back to the object, looked through 
an eye-piece placed on one side of the mouth of the tube, while 
the rays issuing from the object entered at the other side. By 
this means, a second reflexion was avoided ; and though a slight 
distortion in the image must have resulted from the obliquity 
of the first reflexion, yet the inclination of the incident and the 
refletited rays was too small to produce any perceptible error. 
In order to shorten the reflecting telescope, M. Burckhardt 
of the Academy of Sciences proposed to substitute a plane spe- 
culum in place of the small concave speculum of the Gregorian 
telescope. By giving the plane speculum half the diameter of 
the large one, and placing it half-way between the large spe- 
culum and its principal focus, it intercepted the cones of rays 
at the middle of their axis, and reflected them to foci within the 
perforation of the great speculum In extending this princi- 
ple to the Newtonian telescope, M. Burckhardt proposed that 
the plane speculum (of the same size, and similarly placed, as 
in the preceding construction), should be inclined to the reflected 
lays, so as to throw the image above the great speculum, where 
it is viewed by the eye-glass. 
The disadvantages of these constructions are very obvious, 
and are by no means compensated by the trivial advantage of 
shortening the telescope. The difficulty of giving a good figure 
to a plane speculum one-half the diameter of the large one, — 
the loss of light by a perpendicular reflexion, — and the circum- 
stance of the plane speculum intercepting such a large portion 
of the incident rays, will prevent this construction from being 
carried into effect f. 
• This construction was proposed by Janoies Gregory himself so early as 1672. 
See David Gregory’s Elements of Catoptrics and Dioptrics^ 2d edit. p. 261 , 262. 
*j* * Mr Paterson had previously constructed a Newtonian telescope upon the very 
same principle as that of M. Burckhardt ; but with this difference, that the plane 
speculum was placed at one side, so as not to intercept any of the incident rays, 
and consequently the great speculum inclined to the incident rays. This, however, 
cannot be considered as an improvement ; for, though it removes th? objection aris- 
ing from the loss of lights it introduces the much greater evil of an oblique reflexion, 
and consequently a distorted image. In telescopes of 5 feet focal length, and an 
