6q4 philosophical transactions. [anno 1672. 



As to the objection, that with this kind of perspectives, objects are found with 

 difficulty, he answers in another letter, of Jan. 6, 1644, that this is the incon- 

 venience of all tubes that magnify much; and that after a little use the incon- 

 venience will grow less, seeing that himself could readily enough find any day- 

 objects, by knowing which way they were posited from other objects that he 

 accidentally saw in it ; but in the night to find stars, he acknowledges it to be 

 more troublesome, which yet may, in his opinion, be easily remedied by two 

 sights affixed to the iron rod by which the tube is sustained ; or by an ordinary 

 perspective glass fastened to the same frame with the tube, and directed towards 

 the same object, as Descartes in his dioptrics has described, for remedying the 

 same inconvenience of his best telescopes. 



So far the Inventors letters touching this instrument ; of which having com- 

 municated the description to M. Christian Huygens, we received from him an 

 answer to this effect, in a letter of Feb. 13, 1672, N. S 



I see by the description you have sent me of Mr. Newton's admiraoic teles- 

 cope, that he has well considered the advantage which a concave speculum has 

 above convex glasses in collecting the parallel rays, which certainly, according 

 to the calculation I have made, is very great. Hence it is that he can give a 

 far greater aperture to that speculum, than to an object-glass of the same dis- 

 tance of the focus, and consequently that he can magnify objects much more 

 this way than by an ordinary telescope. Besides by it he avoids an inconveni- 

 ence which is inseparable from convex object-glasses, which is the obliquity of 

 both their surfaces, which vitiates the refraction of the rays that pass towards 

 the sides of the glass, and does more hurt than men are aware of. Again, by 

 the mere reflection of the metalline speculum there are not so many rays lost, as 

 in glasses which reflect a considerable quantity by each of their surfaces, and 

 besides intercept many of them by the obscurity of their matter. 



Mean time the main business will be, to find a matter for this speculum that 

 will bear so good and even a polish as glasses, and a way of giving this 

 polish without vitiating the spherical figure. Hitherto I have found no specula 

 that had near so good a polish as glass; and if M. Newton has not already found 

 a way to make it better than ordinarily I apprehend, his telescopes will not so 

 well distinguish objects as those with glasses. But it is worth while to search 

 for a remedy to this inconvenience, and 1 despair not of finding one. I believe 

 that M. Newton has not been without considering the advantage which a para- 

 bolical speculum would have above a spherical one in this construction ; but 

 that he despairs, as well as I do, of working other surfaces than spherical ones 

 with due exactness ; though else it be more easy to make a parabolical than 

 eliptical or hyperbolical ones, by reason of a certain propriety of the parabolic 



