Lord Oxtnantown on a new Reflecting Telescope. 27 



fourth of an inch diameter, accurately perpendicular to their 

 surfaces. The two plates were then unscrewed, and the holes 

 in the plate CD were carefully tapped with a tap, having one- 

 sixtieth of an inch interval between its threads. Three cast 

 steel spindles were then accurately turned, the shank EF being 

 made to fit the holes of the plate AB, and the screw GH 

 nicely to fit the holes in the plate CD. It is almost needless 

 to observe that the flanches, and indeed every part of the 

 spindles, must be very carefully turned. The three cast steei 

 spindles were then put through the holes in the plate AB, till 

 their flanches rested on it. They were secured there by wash- 

 ers IK, put on the shanks of the spindles at the back of the 

 plate, and the washers were retained in their places by milled 

 nuts LM. To prevent the washers from shaking as the steel 

 spindles were turned backwards and forwards, which would 

 loosen the milled nuts, each washer was provided with a screw 

 in its side O, which enters the groove P in the shank, and 

 keeps it steady. The plate CD was then laid upon the three 

 screws GH of the spindles, which were then gradually turned 

 round in succession by a key fitting their other ends till the 

 plate CD reached within about the one-eighth of an inch of 

 their flanches. To prevent the spindles from shaking either 

 in the plate AB, or CD, lateral holes were drilled reaching the 

 principal holes 0,0,0, v 9 v,v. These were stuffed with small bitspf 

 leather, which were kept constantly pressed against the spindles 

 by small screws. This precaution is essential. Besides, screws 

 were inserted at the back of the plate through the holes mark- 

 ed i i i, which were screwed against the plate CD, to prevent 

 the possibility of its shaking in the slightest degree during the 

 operation of grinding and polishing the speculum. 



The mechanism being now complete, a speculum was cast one 

 inch thick. This was secured to the plate CD by three small 

 screwed wires cast into it, by a groove in the plate CD, and 

 by a cement composed of resin, wax, and sulphate of lime. 

 A ring of speculum metal was also cast one inch and a-half thick, 

 which was secured to the plate AB in a similar manner, leaving 

 a very minute interval between it and the piece of speculum 

 metal it surrounded. The whole together formed a speculum 

 of six inches aperture, and two feet focal length, which was 



