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 
Fic. 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- 
