VOL. XLVIII,] PHILOSOPHICAL TRANSACTIONS. 363 



diameter dm, equidistant from, and parallel to it. The distance of these two 

 lines ao, eq, from each other equal to the kerf of the saw, which divided the 

 mirror. The diameter of this plate must be equal to that of the mirror before 

 it was divided. 



On the under side of the plate must be two pins fastened t, t, their diameters 

 equal to the kerf of the saw, that they may keep the two frustums of the mirror 

 at the same distance from each other that they were before their division ; so 

 shall their circular edges be extended as far as the circumference of the plate, 

 and their straight edges touch the said pins in the lines ao, eq. 



The end of the tube must be turned on the inside exactly to fit the plate and 

 mirror, that they may not slide any way, for that would spoil the observations. 



In the diameter of the plate rs, on the points v, w, distant half an inch from 

 c, the centre of the plate, and a whole inch from each other, let a circle, for 

 the aperture of each frustum, of a proper size, according to the intended charge 

 of the telescope, be described, and cut out. Also in the said diameter, equidis- 

 tant from the centre c, viz. at x and z, let there be a screw for each frustum, 

 to elevate it a little from the plate, as shall be needful. Let there be a spring 

 contrived to press on the hack of the one frustum ora, against the point v, being 

 the middle between the edge ao, and the screw x, to keep the frustum close to 

 the plate at the points a, o, and also close to the screw x, when it is screwed in. 

 Let the like be also done on the back of the other frustum esq. 



Then, 1 , before the two screws are put in at x, z, the two frustums of the 

 mirror will lie plain on the plate of brass, and have one pole at c common to 

 them, and consequently will collect all rays which, during their incidence, are 

 parallel to the axis of the tube, to one common focus in the said axis of the tube, 

 just as they would have done before the mirror was divided. 2. But when the 

 two screws xz are put in their places, and screwed a little way through the brass 

 plate, they will lift the two frustums free from the plate at their circular edges, 

 viz. at r and s, while their straight edges ao, eq, are kept to touch the plate 

 with both their ends (not in the middle, by reason of the mirror's concavity) by 

 the pressure of the springs, as mentioned above. By this means the pole c of 

 the frustum ora, will be removed from c towards r, and likewise the pole c of 

 the other frustum esq be removed from c towards s, more or less according to 

 the quantity of the elevation of each frustum, by the screw that raises it; so 

 that now there will appear at the focus two solar images; whereas there was but 

 one, before the screws were put in. 



By moving the screws, the two solar images may be brought to any distance 

 from each other; but care must be taken not to raise one frustum more than 

 the other, and the two solar images nuist almost touch one another at the time 

 of the perigee ; otherwise it must be better adjusted. ., 



3 A ii 



