252 Principle of tbe felefcope. [Book III. 



nomers, and for objects on land feveral contrivances 

 are ufed to rectify this appearance. 



The quantity of the object vifible depends upon the 

 rr.riiitude of the eye-glafs. Let Ap reprefent an ex- 

 treme ray refracted by the object-glafs j if the e'ye-glafs 

 is of fuch a magnitude as to intercept it, the whole 

 of the object will be feen ; if the eye-glafs is too fmall 

 for this purpofe, join D A, C B, the extremities of 

 the lenfes, and the part of the image cut off by thefe 

 lines will mew the proportional part of the object 

 which is invifible. 



Since the focal lengths of two lenfes are fufceptible 

 of any proportion whatever, it might feem that no- 

 thing more was necefiary than to take two lenfes of 

 determinate focal lengths to make us intimately ac- 

 quainted with the mcft diliant of the heavenly bodies; 

 but after a certain length the difficulty of managing 

 thefe glafies becomes infurmountable ; and for diftant 

 objects on the earth, when the magnifying power is 

 more than a hundred, the vapours on the earth would 

 render vifion obfcure. 



The breadth of the object-glafs is of no confequence 

 as to the magnifying power, for whatever it may be, 

 the image will be equally formed at the diftance of 

 its focal length ; but the brilliancy of the image will 

 be increafed by the breadth, as a greater number of 

 rays will then diverge from every given point of the 

 image. 



To mnke the image appear erect, two other con- 

 vex lenfes are required, of equal focal lengths. The 

 rays emerging from the eye-glafs are intercepted by 

 the firft of thefe lenfes, and are made to converge to 

 points at the diftance of its focal length j thence they 

 diverge, and being intercepted by the other lens at the 

 diftance of its focal length, they are made to proceed 



parallel 



