a62 PHILOSOPHICAL TRANSACTIONS. [anNO 1753. 



plate there must he made, in the middle of each tube, a round aperture, viz. 

 hg, \vx, whose diameters must be proportioned to the focal length of the eye-lens, 

 and not exceed the third part of it, lest the object appear confused. 



And since it is scarcely possible to centre an object-lens to very good perfec- 

 tion, those in the two cylinders, may happen to render the two solar images at 

 too great a distance from, or too near to each other. But this fault, if not too 

 great, may be remedied, by turning one or both of the lenses a little way round; 

 and then their eccentric poles will by that means be brought nearer to, or farther 

 from each other; and when they are oncp well placed, there should be a mark 

 made in each lens, and its cylinder; that if it is taken out to be wiped, it may 

 be put in again the same way. There should also be a different mark in one of 

 the glasses, that each may know its own cylinder. They must both of them be 

 very close all round to their respective cylinders; otherwise one lens may slide 

 nearer to or farther from the other; which if it should in the least degree, be- 

 tween the first and second observation, all the labour would be lost. Either of 

 these three parts of double lenses may be combined with a convex eye-lens as 

 usual, and have a micrometer placed at the common focus. 



Such a double lens, of either sort, may be proved whether it is well composed 

 or not, without the trouble of combining it with its eye lens, bv holding it in 

 the sun's rays, as one would a burning glass, and applying a piece of white paper 

 ' at its focus, where, he apprehends, the two solar images will appear as distinct 

 as when an eye-lens is applied, though not so large; and each of them 1 inch 

 broad, if the focal length be as above, i. e. almost 104-,25^ inches. After the 

 same manner may the double object-mirror of a reflecting telescope for this use 

 be proved. 



In fig. 6, the circle bdhpmg is the circumference of a concave mirror made of 

 black glass: it must be very thick, that it may not spring or bend with any thing 

 that presses on it to keep it fast, for that may injure its concavity. The circle 

 within it, on the same centre c, shows that its concavity must not be continued 

 quite home to the very edge of the mirror, but the little space between the two 

 circles must be ground very true on a plain. The pricked lines must not be 

 drawn; they are only to indicate where the poles vw of the two frustums must 

 be brought, after the mirror is diametrically bisected. 'Let the concave side be 

 defended, by pasting a paper all over it, and then let it be divided with a saw in 

 the diameter dcm ; taking care that the said diameter be in the middle of the 

 kerf, which may be as broad as the space between the lines ao, eq. Let the 

 asperities of the edges of both frustums be ground off, that they may be very 

 straight after being sawed. 



Fig. 7j represents a thick round plate of brass, very plain, and equally thick 

 all over, having lines drawn on it, as on fig. 2, also one line on each side of the 



