Vision and Optical Glasses. 193 



What microscopes effect upon minute bodies 

 very near, telescopes effect with regard to great 

 bodies very remote; namely, they enlarge the 

 angle in the eye under which the bodies are seen ; 

 and thus, by making them very large, they make 

 them appear very near: the only difference is, 

 that in the microscope the focus of the glasses is 

 adapted to the inspection of bodies very near ; in 

 the telescope, to such as are very remote. Sup- 

 pose a distant object at A B (see fig. 73), its rays 

 come nearly parallel, and fall upon the convex 

 glass cd; through this they will converge in points, 

 and form the object E at their focus. But it is 

 usually so contrived, that this focus is also the 

 focus of the other convex glass of the tube. The 

 rays of each pencil, therefore, will now diverge 

 before they strike this glass, and will go through 

 it parallel ; but the pencils all together will cross 

 in its focus on the other side, as at e, and the 

 pupil of the eye being in this focus, the image 

 will be viewed through the glass, under the angle 

 geh, so that the object will seem at E under the 

 angle DeC. This telescope inverts the image, 

 and therefore is only proper for viewing such 

 bodies as it is immaterial in what position they 

 appear, as the sun, the fixed stars, &c. By add- 

 ing two convex glasses, the image may be seen 

 upright. The magnifying power of this, which is 

 called the dioptric telescope, is found by dividing 

 the focal distance of the object-glass by the focal 



VOL. I. 



