185 



The center of the great mirror ought to be in the com- 

 mon axis of the instrument; and the position of it in its 

 cell, in the tube, may be known thus. Let the little mii-- 

 ror be taken out of the tube; and let the round, central 

 diaphragm, before-mentioned, (which ought to be made 

 of a flat piece of tinned plate, or a brass plate, made 

 clean and bright enough, to reflect the light sti'ongly, but 

 not polished,) be fastened across the mouth of the tube, 

 exactly in the middle of it; and let a round hole be 

 made through the plate, the center of which shall be in 

 the axis of the tube, and its diameter so large, as that 

 the whole disc of the sun may be viewed through it, at 

 the eye-hole of the telescope, when the eye-glasses are 

 taken out. Then, directing the instrument to the sun, or 

 the full moon, when very bright, so as that its whole disc 

 shall be seen through the hole in the diaphragm; (using a 

 lightly tinged screen-glass, to look at the sun;) if the 

 light, reflected from the great mirror to the diaphragm, 

 occupies on it, a circular area, concentrical AV^ith the hole 

 made at its center, the mirror is rightly placed, and its 

 focus is in the axis of the tube. But, if the edge of the 

 illuminated circle approaches nearer to the hole in the 

 plate, on any side, the same side of the mirror inclines 

 towaixl the axis of the tube, its cell not being exactly 

 vertical to it; or otherwise, the center of the mirror is 

 not in that axis, as it ought to have been. If the outer 

 one, of the thr«e atbresaid diaphragms, be, at the same 

 time, applied to the mo^th of the tube, so as to expose 



only 



