498 French National Institute. 



focus of the great minor, z. e. towards the upper extremity 

 of the tube, which renders it inconvenient to observe under 

 many circumstances, particularly in large telescopes, he 

 places it perpendicularly to the axis, and towards the half of 

 its length. In this place the section of the reflected cone of 

 light is a circle, the diameter of which is precisely the half 

 of that of the great mirror : it will therefore intercept a 

 fourth part of the direct rays ; but M. Burckhardt remedies 

 this loss by giving a larger dimension to the first mirror. 

 The retrenched cone assumes a reverse position : the rays, 

 instead of uniting, as they would have done, beyond the 

 plain mirror, are collected at an equal distance, but in front, 

 and pass through an aperture made in the centre of the great 

 mirror, in the space which, as we have seen, receives no 

 direct ray, and which is consequently useless for assisting 

 vision. The advantage of this construction consists in re- 

 ducing the length of the telescope one half, which thereby 

 becomes easier to manage, and less costly. If the diameter 

 of the concave mirror is a little larger, the central part which 

 should have a hole requires no trouble; it is sufficient that 

 the speculum, the only useful part, should receive the cur- 

 vature necessarv for the distinctness of the image; and when 

 it was really a little difficult to render it very exact, we might 

 make up for it, since we have only a single mirror to curve, 

 and because the plain mirror, on account of its dimension 

 being a little larger than in the Newtonian telescope, fur- 

 nishes easier and more precise verifications. The observer 

 should be placed at the lower part and behind the great mir- 

 ror, as with Gregory's telescope, which is the most con- 

 venient position for following a star continually changing 

 its place. Finally, M. Burckhardt has calculated, by setting 

 out froiTLthe measurements of Newton himself, that a tele- 

 scope of eight metres in focal length, reduced in this way to 

 the actual length of four metres, would have three times 

 more light than a common telescope of four metres, and 

 would have a very valuable advantage over the latter for 

 micrometrical measurements, on account of the double di- 

 stance of its focus. 



Before putting his new idea into execution, M. Burck- 

 hardt 



