FUNCTION AND PARTS OF THE MICROSCOPE. 9 



formed. They will appear to the eye to come from a 

 point more remote than their true position; and the 

 exact location of this point may be found by prolonging 

 the lines from o and F backward until they meet. This 



Fi(5. 8. — Formation of Image by Object inside the Principal 

 Focus. (After Hager-Mez.) 



image will not be inverted, but erect; and as it cannot be 

 caught upon a screen it is called a virtual image. 



7. Development of the Simple Microscope.— The effect 

 of globes of crystal in concentrating the sun's rays on a 

 single point was known in very early times. Aristophanes, 

 Pliny the Elder, and other Greek and Roman authors 

 mention the use of such primitive burning-glasses. Sen- 

 eca states that "letters though small and indistinct are 

 seen enlarged and more distinct through a globe of glass 

 filled with water." None of the ancients appear, how- 

 ever, to have thought of any practical application of this 

 phenomenon as an aid to- vision; and medical writers 

 up to the thirteenth century of the Christian era speak of 

 short-sightedness as an incurable infirmity. 



In microscopy, as in so many other branches of knowl- 



