VOL. XXXII.] ^ PHILOSOPHICAL TRANSACTIONS. 647 



at other times it is closed with a sliding shutter. The metal is placed so, as to 

 have its axis coincide with that of the tube, by the means of three small buttons 

 fixed to the inside of the tube, having their hinder ends all in the same plane, 

 to which this axis is perpendicular. Two of these appear at aa, the third, being 

 at the middle of the bottom of the tube, is not seen. The foreside of the 

 metal rests against these buttons in three points of its circumference, nearly 

 equidistant from each other, and is held to them by three screws, one of which 

 appears at b, which run through the octangular board at the end of the tube, 

 and bear against the back of the metal in three points, which directly answer 

 those three on the foreside, with just so much force as is requisite to keep it 

 steady in its place. 



The oval plane is composed of a plate of the same metal with the great spe- 

 culum, about ^ or tV of an inch in thickness, soldered on the back to another 

 of brass. Its breadth is something less than half an inch, and is in proportion 

 to its length as 1 to s/ 1. At one end of the oval, the brass plate projects a 

 little beyond the other, and has a screw cut through it in that part, as also 

 another directly against the centre of the foreside. The other end is cyphered 

 away on the backside, that it may intercept as few as possible of the rays, in their 

 passage towards the object metal. I'he two screw holes in the back serve to fix 

 this oval A, fig. 12, pi. 15*, to a brass arm b, which is fastened at the other end 

 into a slider ee, fig. 1 1 and 12. This slider is of an equal thickness with the side 

 of the tube, and has a groove gg, fig. 13, cut for it in that side, parallel to the 

 axis, and long enough to give room for its motion, to set the two specula at 

 the different distances, which the several eye-glasses require. It rests on the 

 inside against two thin ledges, fastened within the tube along the sides of the 

 groove. On the outside it is kept in its place by a sliding shutter, not expressed 

 in the figure. In the middle it has a cylindric cavity d, fig. 12, whose axis is 

 exactly perpendicular to its inner and outer surfaces. Each of the boxes, in 

 which the eye-glasses are contained, is fitted to this cavity. The beforemen- 

 tioned brass arm is fixed into the inside of this slider, towards the end farthest 

 from the object metal ; it rises perpendicular for about 2 inches, and is made 

 fiat, so as to turn one edge to the rays which come from the object. About 

 b, it is bent forwards and flatted the other way, so that when the back of the 

 oval plane is held flat to it, by the two screws cc, the axis of the cylindric 

 cavity may fall on the centre of its foreside, inclined to its surface in an angle 

 of something less than 45*^. This angle is brought to be exact by two very 

 small screws ii, whose threads take hold in the flatted end of the brass arm, 

 and their points bearing against the back of the oval, raise one end of it a little 



* At line 21 of the opposite page, for fig. \, pi. 17, read fig. 11, pi. 15. 

 VOL. VI. , ,^ 4n4 



