712 PHILOSOPHICAL TRANSACTIONS. [aNNO 167*2. 



as to its convexity, that it reflects the species which it receives from the great 

 speculum, towards the hole E, where is an eye-glass, which one looks 

 through. 



The advantage which I find in this instrument above that of Mr. Newton, is 

 first, that the mouth or aperture AB of the tube may be of what size you 

 please; and consequently you may liave many more rays on the concave specu- 

 lum, than upon that, of which you have given us the description. — 2. The re- 

 flection of the rays will be very natural, since it will be made upon the axis it- 

 self, and therefore more vivid. — 3. The vision of it will be so much the more 

 pleasing, as you will not be incommoded by the great light, by reason of the 

 bottom CD, which hides the whole face. Besides that you have less difficulty 

 in discovering the objects than in Mr. Newton's, 



The following are the Coruiderations of Mr. Newton, as we received them from 

 him in a Letter, written from Cambridge, May 4, 1672. 



Sir — I should be very glad to meet with any improvement of the catadiop- 

 trical telescope ; but that design of it^ which (as you inform me) Mr. Cassegrain 

 has communicated 3 months since, and is now printed in one of the French 

 memoirs, I fear will not answer expectation. For when I flrst applied myself 

 to try the effects of reflections, Mr. Gregory's Optica Promota (printed in the 

 year 1663) having fallen into my hands, where there is an instrument (described 

 in page 94) like that of M. Cassegrain's, with a hole in the midst of the object 

 metal, to transmit the light to an eye-glass placed behind it ; I had thence an 

 occasion of considering that sort of constructions, and found their disadvantages 

 so great, that I saw it necessary, before I attempted any thing in the practice, 

 to alter the design of them, and place the eye-glass at the side of the tube, 

 rather than at the middle. 



The disadvantages of it you will understand by these particulars. — 1. There 

 will be more light lost in the metal by reflection from the little convex specu- 

 lum, than from the oval plane. For it is an obvious observation, that light is 

 most copiously reflected from any substance when incident most obliquely. — 

 2. The convex speculum will not reflect the rays so truly as the oval plane, 

 unless it be of an hyperbolic figure; which is incomparably more difficult to 

 form than a plane; and if truly formed, yet would only reflect those rays 

 truly which respect the axis. — 3. The errors of the said convex will be much 

 augmented by the too great distance, through which the rays reflected from it 

 must pass before their arrival at the eye-glass. For which reason I find it con- 

 venient to make the tube no wider than is necessary, that the eye-glass be 

 placed as near to the oval plane as is possible, without obstructing any useful 

 light in its passage to the object metal. — 4. The errors of the object metal will 



