260 FERGUSON'S LECTURES. 



LECT. feet focal distance, will admit of an eye-gin ss whose 



^!^ focal distance is little more than 2i inches ; which will 



magnify nearly 48 times : but an objecl-slao* ^f 100 ifcet 



t :u . .^MHX, an e^e-giass somewhat more than 6 



inches ; and will therefore magnify almost 200 times. 



A telescope for viewing terrestrial objects should be 

 so constructed as to shew them in their natural posture. 

 And this is done by one object-glass c d, and three eye- 



glasses ef, gh, ik, so placed that the distance between 

 any two, which are nearest to each other, may be equal 

 to the sum of their focal distances ; as in the figure, 

 where the focus of the glasses c d and e f meet at F, 

 those of the glasses ef and g h, meet at /, and of g h and 

 i k, at m ; the eye being at 11, in or near the focus of the 

 eye-glass ik, on the other side. Then, it is plain, that 

 those pencils of rays, which flow from the object A B, 

 and pass through the object-glass cd, will meet and form 

 an inverted image CFD in the focus of that glass; 

 and the image being also in the focus of the glass ef, 

 the rays of the pencils will become parallel, after pass- 

 ing through that glass, and cross at /, in the focus of 

 the glass ef; from whence they pass on to the next 

 glass g h, and by going through it they are converged 

 to points in its other focus, where they form an erect 

 image EmF, of the object AB: and as this image is 

 is also in the focus of the eye-glass ik, and the eye on 

 the opposite side of the same glass ; the image is viewed 

 through the eye-glass in this telescope, in the same 

 manner as through the eye-glass in the former one ; 

 only in a contrary position, that is, in the same position 

 with the object. 



The three glasses next the eye, have all their focal 



