186 



.only the middle zone of the great mirror to the light; 

 -the circle of light, reflected by it, to thie central dia- 

 phragm, will appear better defined on it. 

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

 the same time, be. proved, by the following most easy 

 •and certain method, if exactly pursued. 



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

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

 ■the great tube; and proved that it is so, as Mr. Ed- 

 wards prescribes, 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 that mirror is screwed, concentrical Mith the 

 plate, cori-esponds with the intersectioa of two cross hairs, 

 tied diametrically across the mouth of the tube: then 

 let the little mirror be replaced, and the eye-tube taken 

 out, and let tlie telescope be directed to the sun's cen- 

 ter; which may be .done, by the help of the little diop- 

 trical telescope, called a finder, affixed to the instrument, 

 if it be furnished Avith such; or otherwise, may be effect- 

 ed, with siifficient exactness for this purpose, by point- 

 ing the telescope to the sun, so as that the shadow of 

 the little mirror may coincide Avith the hole, in the great 

 mirror; which will be, when the great tube is so placed, 

 as to project its shadow of a circular form. In these cir- 

 cumstances, let the light, reflected from the little mirror, 

 through the round perforation in the great one, be re- 

 ceived upon a plane, plac(>d at some distance behind the 



latter. 



