19^ PHILOSOPHICAL TRANSACTIONS. [ANNOI667. 



bered by 10, 20, 30, &c. Through the middle of this plate, and the middle 

 of the box a a ay is placed a very curiously wrought screw, about the size of a 

 goose quill, and of the length of the box, the head of which is, by a fixed ring 

 or shoulder on the inside, and a small springing plate dd on the outside, so 

 adapted to the plate that it is not in the least subject to shake. The other end 

 of this screw is, by another little screw (whose small point fills the centre or 

 hole made in the end of the longer screw for this pui-pose) rendered so fixed 

 and steady in the box, that there appears not the least danger of shaking. On 

 the head of this screw, without the springing plate, is put on a small index e e, 

 and above that a handle m m, to turn the screw round as often as there shall 

 be occasion, without at all endangering the displacing of the index ; it being 

 put on very stiff on a cylindrical part of the head, and the handle on a square. 

 The screw has that third part of it which is next the plate larger than the other 

 two thirds of it, by at least as much as the depth of the small screw made on 

 it : the thread of the screw of the larger third is as small again as that of the screw 

 of the other two thirds. To the thicker screw is adapted a socket y, fastened to 

 along bar or bolt g g, on which is fastened the moveable sight h, so that every 

 turn of the screw moves the sight h either a thread nearer or a thread farther 

 off from the fixed sight i. The bar jg" ^ is made exactly equal, and fitted into 

 two small staples k k, which will not admit of any shaking. There are 6o of 

 these threads, and answerable thereto are made 6o divisions on the edge of the 

 bolt or ruler g g ; and a small index /, fixed to the box a a a, denotes how many 

 threads the edges of the two sights h and / are distant ; and the index e e shows, 

 on the circular plate, what part of a revolution there is more ; every revolution, 

 .as was said before, being divided into 100 parts. At the same time that the move- 

 able sight h is moved forwards or backwards, or more threads of the coarser 

 ■screw, is the plate pj&, in fig. 2, by the means of the socket q, to which it is 

 screwed, moved forward or backward, or more threads of the finer screw : so 

 that this plate being fixed to the telescope by the screws r r in fig. 2, so as the 

 middle between the sights may lie in the axis of the glass, however the screw 

 be turned, the midst between the sights will always be in the axis, and the 

 sights will equally either open from it or shut towards it. 



Fig. 2 represents the moveable cover containing the screws to be fitly placed 

 on fig. 1 ; according to the taking off, as it were, or folding up of this cover, 

 the inward contrivance of the screws and sights may appear. 



And because it is conceived by some ingenious men, that it will be more 

 convenient, instead of the edges of the two sights h and /, to employ two 

 sights fitted with hairs, therefore is added fig. 3, representing the two sights 



