(INTERMEDIATE TERM.) 



ELEMENTARY TECHNOLOGY. 



I. THE SIMPLE MICROSCOPE. 



1. Lenses. 



Ancient Assyrian lens from Sargon's palace, Nimroud, B. C. 

 721. 



Two ancient glass bosses in British Museum, B. C. 270 con- 

 troverted. 



Pliny speaks of glass globes used as burning glasses 

 minutely engraved gems etc. Yet essentially the microscope 

 is modern. Spectacles for old men referred to A. D. 1299. 

 From Malpighi in 1661 to Di Torre 1771, the most ingenious 

 plans were made to construct and use very small lenses. The 

 discoveries of Lewenhceck, Hook, Lieberkuhn, Hewson, etc., 

 were made by patient industry with these, often constructed by 

 themselves. Different kinds of lenses, Fig. I, PL I. 



2. Optical principles. 



Optical principles few and simple depend on laws of light 

 Law a mode or order of things. Nature of light not needed 

 to be known, but laws. 



1. Rays from a luminary go in straight lines. 



2. Rays from a rarer to a denser medium are refracted 

 (bent) towards a perpendicular, and vice versa. 



3. The sines of the angles of incidence and of refraction are 

 a constant ratio for each substance (refractive index). 



4. The angles of incidence and reflection are equal. 



In Fig. 2, PL i, A B is the surface of denser medium, C D 

 the perpendicular, E F refracted ray, G reflected ray, a b 

 sine of angle of incident, a' b' sine of angle of refraction. 



