692 PHILOSOPHICAL TRANSACTIONS. [aNNO 1672. 



with the flat specukim, obhquely posited, by the reflection of which they are 

 directed to the Uttle plano-convex glass, and so to the spectator's eye, who look- 

 ing downwards sees the object which the telescope is turned to. 



To understand this more distinctly and fully, the reader will observe that AB 

 is the concave speculum, of which the radius or semi-diameter is 12-f- or 13 

 inches. CD another metalline speculum, whose surface is flat and the circum- 

 ference oval. GD an iron wire, holding a ring of brass, in which the speculum 

 CD is fixed. 



F, a small eye glass, flat above, and convex below, of the 12th part of an inch 

 radius, if not less; as the metal collects the sun's rays at 6-^ inches distance, and 

 the eye-glass at less than -^ of an inch distance from its vertex : besides that the 

 author knew their dimensions by the tools to which they were ground, and par- 

 ticularly measuring the diameter of the hemispherical concave, in which the eye 

 glass was wrought, found it the 6th part of an inch. 



GGG, the fore part of the tube, fastened to a brass ring HI, to keep it im- 

 moveable. PQKL, the hind part of the tube, fastened to another brass ring 

 PQ. O, an iron hook, fastened to the ring PQ, and furnished with a screw N, 

 thereby to advance or draw back the hind part of the tube, and so by that means 

 to put the specula at their due distance. MQGI, a crooked iron, sustaining 

 the tube, and fastened by the nail R to the ball and socket S, whereby the tube 

 may be turned every way. The centre of the flat speculum CD, must be placed 

 in the same point of the tube's axis, where falls the perpendicular to this axis, 

 drawn to the same from the centre of the little eye-glass, which point is here 

 marked at T. 



And that the reader may understand in what degree it represents things dis- 

 tinct, and free from colours, and know the aperture by which it admits light, 

 he may compare the distances of the focus E from the vertices of the little eye- 

 glass and concave speculum, that is, EF, -^ of an inch, and ETV, 6j- inches, and 

 the ratio will be found as 1 to 38; whereby it appears that the objects will be 

 magnified about 38 times. To which proportion is very consentaneous an 

 observation of the crown on the weather-cock, about 300 feet distant. For the 

 scheme X, figure 2, represents it larger by 2-i- times in diameter, when seen 

 through this, than through an ordinary telescope (as at figure 3) of about two 

 feet long. And so supposing this ordinary one to magnify 13 or 14 times, as 

 by the description it should, this new one by the experiment must magnify 

 near as much as has been assigned. 



Thus far as to the structure of this telescope. Concerning the metalline 

 matter fit for these refl.ecting speculums, the inventor has also considered the 



