"98 REFLECTING TELESCOPE. 



Directions for '^^^ Centre of the great mirror ought to be In the corn- 

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

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

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

 phragm, 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 strongly, 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 centre of which shall be in the axis of the tube, 

 and its diameter so large, as that the whole disc of the sun 

 maybe 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, concentricai 

 withthe hole made at its centre, 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 to- 

 ward the axis of the tube, its cell not being exactly vertical 

 to it; or otherwise, the centre of the mirror is not in that 

 axis, as it ought to have been. If the outer one, of the 

 three aforesaid diaphragms, be, at the same time, applied to 

 the mouth of the tube, so as to expose only the middle zone 

 of the great mirror to the light ; the circle of light, re^ 

 fleeted by it, to the central diaphragm, will appear better 

 defined on it. 

 Thespeculums But the adjustment of both mirrors and lenses may, at 

 may be very the same time, be proved, by the following most easy and 

 S r? s^'^^ certain method, if exactly pu'rsued : 



Having provided, that the little mirror shall be so sup- 

 ported, that its centre may always move in the axis of the 

 great tube; and proved that it is so, as Mr. Edwards pre- 

 scribes, by taking off" the mirror, and seeing, through the 

 eye-tube of the telescope, (without the lenses.) that the 

 hole, in the middle of the little round plate, to which hat 

 mirror is screwed, concentricai with tlic pl.-Ue, correspt nds 

 with the intersection of two cross hairs, tied diametrically 



across 



