THE MICROSCOPE. 
mites in oatmeal, &.; (2) Dr. Robert Hooke, who shaped the smallest 
of his lenses from small glass globules made by fusing the ends of 
threads of spun glass. He published his Micrographia Illustrata in 
1625, in the Philosophical Transactions, describing his invention of 
magnifying glasses of immense power. 
Such simple lenses as Leeuwenhoek and Hooke made are now exem- 
plified by the pocket lens or hand magnifier, and the magnifying or 
reading glass. They produce an enlarged erect image of an object 
PROBOSCIS OF BLOWFLY. SHOWING SPIRACLES FOR AIR SUPPLY. 
Fifty Times Natural Size. 
that is placed between the principal focus and the lens, appearing 
further removed from the lens than the object itself. These lenses, 
when used as burning glasses, focus the rays of the sun to a central 
point called the principal focus, and in such simple lenses the length 
from the optical centre to the foctis can be readily measured. (The 
optical centre in this case is midway between the two curved faces. 
-When the lens is very thick, or where several lenses are combined together, 
290) 
