648 PHILOSOPHICAL TRANSACTIONS. [aNNO 1723. 



from the flat of the arm. The specula are set at their due distance, by turning 

 a long screw cc, for which there is a nut lodged in the slider at g; the screw 

 is kept from moving backward or forward, when turned, by a brass plate, f, 

 which is to be fixed to the flat end of the side of the tube, and taken off at 

 pleasure. Each of the eye-glass boxes, h, has a screw on the outer end, to 

 fasten to it a bowl or dish, i, to receive the ball of the eye, and guard it from 

 external light. 



On the top of the tube, on two small pedestals, is fixed a common dioptric 

 telescope, h, fig. 1 1, about 18 inches long, its axis parallel to that of the tube; 

 and having two hairs placed in the common focus of its object and eye-glasses, 

 crossing each other in its axis. 



There are three convex eye-glasses belonging to the instrument. The first, 

 or shallowest, has its focal distance of about 4- of an inch; the second of -^; 

 and the deepest of 44-j or something less. When the first of these is used 

 with the instrument, it magnifies about 188 or IQO times in diameter; with 

 the second, about 208 ; and with the third, 228 or 230. Each of these glasses 

 has placed, in that focus nearest the oval, a circle to determine the part of the 

 object seen at one view ; and in the other focus toward the eye, a brass plate 

 with a little hole in the middle, to let no light pass to the eye from the inside 

 of the tube, but what comes from the oval. Besides these three convex, there 

 are two concave eye-glasses, with which it magnifies about 200 and 220 times; 

 and also a set of three convex, which turn it into a day telescope, magnifying 

 about 125 times. The aperture is limited by a circle of card, or pasteboard, 

 placed before the object metal in the tube. To vary the aperture, there are 

 three of these circles, and the apertures allowed by them are 5-|- inches, 5 

 inches, and 44-, though for some objects the whole metal may be left open. 



The engine made use of to direct the tube to any object, consists of a strong 

 plank, FF, fig. 11 and 13, about 14 inches wide, and 2^ feet or 3 feet long, 

 which serves as a foundation for the whole. Near one end of this plank is 

 placed an upright foursided box, iii, fig. 11 and 13, about 2 feet high, nar- 

 rower at the back next the end of the plank than before; its two sides are mor- 

 tised both into the plank below, aa, fig. 13, and into the top of the box above, 

 dd, the back and fore part are fastened to the edges of the sides with wood- 

 screws. The top has a circular hole cut in it, something above 3 inches in 

 diameter, whose centre is about 3 inches distant from the outside of the back, 

 and at an equal distance from the two sides. This hole gives passage to a 

 turning pillar b, in the bottom of which there is fixed an iron pivot, c, to turn 

 in a thick brass plate lodged in the plank, b. The upper end of the pillar rises 

 about an inch and a half above the top of the box, and is mortised into a 

 strong head, k, fig. 1 1 and 13, about 8 inches in length, and 4 or 5 in breadth 



