Aildilional No/e^. 



409 



attend this. But as it m:iy be made so simple, cnsy, and light. 

 Hi not to be much more unwieldy or uuniaiuiijeable, tiioiigli of 

 considerable length, tlian a single telescope ul" the same, that in- 

 conveniency will be abundantly compensated. 



The description of it, as he proposes it, and has got one made, 

 is nearly thus, which he is v illing I should communicate to thee, 

 if possibly it may be of service. 



To a straight ruler or piece of wood, AB, of about three mchea 

 m breadth, and from forty to forty-five in length (or of any other 

 that may be thought convenient), with a suitable thickness; an 

 arch or limb, AC, of about 30 degrees to the radius, KL, is to be 

 fixed. To the upper end of the piece AB, a piece, DD, is to b© 

 morticed, and in it the centre K taken, so that OP may be about 

 six inches, and the angle KOP about 40 degrees. On this centre 

 K, the ruler or ii.'dex KL is to move, having a liducial edge below 



answerable to the central 

 point, to cut the graduations 

 on the limb. On the upj^er 

 end of this index a speculum 

 of silvered glass, or rather 

 metal, exactly plain, EF, of 

 about three inches in length 

 and two in height, is erected 

 perpendicular to the plane 

 of tlie index, and also nearly 

 at right angles with its sides, 

 the plane of the retlectiug 

 surface standing exactly over 

 tlie central point. At the 

 end B, of the piece AB, an- 

 other speculum of glass is 

 to be in the same manner 

 erected, which maybe some- 

 what less than the other, 

 with a square or oblong spot 

 in it unsilvered, tliat a star, 

 by a direct ray, may be seen 

 through it J and the back of 

 til is speculum should be 

 guarded witli a tlilu brass 



y 



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