187 



latter, at right angles to, the axis of the •tube. The Ught 

 will occupy a circular area on the plane; and, if th« 

 center of the lia;ht be coincident with that of the shadow 

 of the little mirror, this mirror is not only parallel to the 

 great one, but both are duly adjusted, at right angles, to 

 the axis of the tube ; which, also, then corresponds with 

 their axes. Buts, because the little mirror and its shadow, 

 and also the cone of light, reflected from this mirror, are 

 of greater breadth than the perforation in the great one;- 

 the boundaries af the reflected light', and those of the 

 shadow, cannot be seen wholly' o&ithe plarte, through the 

 hale in the great mirror, in any one position of the te- 

 lescope. Let, therefore, the axis of the telescope be a 

 little diverted from the center of the sun, till the shadow 

 of the edoe of the little mirror falls within the hole in 

 the great one: by which, some direct light will pass' through, 

 next the shadow, and appear on the plane,- in forni of a 

 crescent; and, at the saime time, the circle of the reflect- 

 ed light of the suii. will have 'moved across the shado^v: 

 till, by a certain degree of^oblrquity, in the direction of 

 t^e -telescope^ the edge' of the iciitcleof the- I'eflected light- 

 will be in contact,' externally, witb the crescent of the di- 

 rect light. And, if the^ crescent) be always, of the same 

 breadth,, when this 'contact takes place, : on; ' eveay side, by 

 la diverting of. the telescopt, 'from the centser of the sun, 

 -successivjsly, in: every, direction; then both {he mirrors are 

 parallel,' 'ami Jiat'-right angles^ to. the axis of the instru- 

 •naent. But,, if .the-i cirescerit be broader, in any certain 

 position of the tube, when the circle of reflfected: light just 

 :..'J touches 



