256* Refletfing Telefcope. [Book III. 



two rays of a pencil flowing from each extremity of 

 the object into the great tube, and trace their pro- 

 grefs through all their reflexions and refractions to the 

 eye /at the end of the fmall tube //, which is joined 

 to the great one. 



Let us then fuppofe the object A B to be at fuch 

 a diftance, that the rays C may flow from its lower 

 extremity B, and the rays E from its upper extre- 

 mity A ; then the rays C falling parallel upon the 

 great mirror at D, will be thence reflected converg- 

 ing in the direction D.G, and by croffing at I in the 

 principal focus of the mirror, they will form the upper 

 extremity. I of the inverted image I K, fimilar to the 

 Jower extremity B of the object A B, and patting on 

 to the concave mirror L (whofe focus is at n) they 

 will fall upon it at g, and be thence reflected, con- 

 verging in the direction gN, becaufe gm is longer 

 than g n y and paffing through the hole P in the large 

 mirror, they would meet fomewhere about r t and from 

 the lower extremity b of the erect image a b, fimilar 

 to the lower extremity B of the object A B. But by 

 pafling through the plano-convex glafs R in their way, 

 they form that extremity of the image at &. In the 

 fame manner the rays E, which come from the top of 

 the object A B, and fall parallel upon the great mirror 

 at F, are thence reflected, converging to its focus, 

 where they form the lower extren ity K of the invert- 

 ed image I K fimilar to the upper extremity A of the 

 object A B, and thence pafiing on to the fmall mirror 

 L, and falling upon it at b, they are thence| reflected 

 in the converging ftate h O ; and going on through 

 the hole P of the great mirror, they would meet fome- 

 where about q> and form there the upper extremity 

 a of the erect image a b t fimilar to the upper extre- 

 mity 



