THE MICROSCOPE. 519 



turned straight towards it, so that the image of the object 

 is formed on a point of the retina which lies at one end 

 of a straight line joining the body and the retina, and 

 traversing a particular region of the centre of the eye. 

 This straight line is called the optic axis. 



The nearer to the eye an object is situated the larger 

 is its image upon the retina, and the larger does it 

 appear to us. The distinctness of the visible object 

 increases also with its apparent size, provided that its 

 image on the retina is sharp and distinct. On an 

 average the greatest distance of distinct vision is about 

 25 cm from the eye ; beyond this distance images of 

 objects appear smaller and therefore less distinct ; at a 

 less distance, on the other hand, the image becomes 

 blurred and the object therefore indistinct. To a short- 

 sighted eye an object is still sharply defined at a less 

 distance than 25 cm , and such objects appear therefore 

 larger and more distinct than others at a greater dis- 

 tance, while a long-sighted eye sees an object distinctly 

 only at distances beyond 25 cm , and the objects appear 

 therefore smaller. 



Microscopes and telescopes are ' optical instruments,' 

 used for making near or distant objects appear larger or 

 more distinct than they do when seen with the naked eye. 

 The lens described on page 475 is a magnifying glass : 

 when the lens is held close to the eye, a magnified erect 

 virtual image is produced of an object placed at the 

 proper distance ; and if the focal length of the lens is 

 known, its magnifying power may be found by adding 

 the 'distance of distinct vision' (25 cm ) to the focal 

 length, and dividing the sum by the latter. Thus a lens 



of 10 cm focal length magnifies - = 3*5 times; 



