264 FERGUSON'S LECTURES. 



LKCT. g, and be thence reflected converging, in the direction 

 ^?\L, g ZV, because g m is longer than g n ; and passing through 

 the hole P in the large mirror, they would meet some- 

 where about r, and form the lower extremity d of the 

 erect image a d, similar to the lower extremity B of the 

 object A B. But by passing through the plano-convex 

 glass R in their way, they form that extremity of the 

 image at b. In like manner, the rays , 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 extremity K of 

 the inverted image / K, similar to the upper extremity 

 A of the object A B ; and thence passing on* to the 

 small mirror L, and falling upon it at h, they are thence 

 reflected in the converging state hO; and going on 

 through the hole P of the great mirror, they will meet 

 somewhere about q, and form there the upper extremity 

 a of the erect image a d, similar to the upper extremity 

 A of the object A B : but, by passing through the con- 

 vex glass R in their way, they meet and cross sooner 

 as at a, where that point of the erect image is formed. 

 The like being understood of all those rays which flow 

 from the intermediate points of the object, between A 

 and B, and enter the tube TT; all the intermediate 

 points of the image between a and b will be formed : 

 and the rays passing on from the image through the 

 eye-glass S, and through a small hole e in the end of 

 the lesser tube 1 1, they enter the eye f, which sees the 

 image ad (by means of the eye-glass) under the large 

 angle ced, and magnified in length, under that angle 

 from c to d. 



In the best reflecting telescopes, the focus of the small 

 mirror is never coincident with the focus m of the great 

 one, where the first image I K is formed, but a little 

 beyond it (with respect to the eye) as at : the con- 

 sequence of which is, that the rays of the pencils will 

 not be parallel after reflection from the small mirror, 



