P T 



OPERA-GLASS. KitcJiiner. 



An Opera-g-lass should not magnify more than three, or at the most 

 four times ; this also makes a pleasant prospect glass. If it have besides 

 a power magnifying twice, it will be an excellent assistant in giving- a 

 general view of the constellations, and will be a good finder for sweep- 

 ing the sky for a comet. The best Opera-glasses at present made by op- 

 ticians, have an achromatic object glass of one and a half inch in diame- 

 ter, magnifying four times ; the price in a plain mounting, about two 

 guineas and a half. 



OPTICS, laws of. 



The theory of Optics reposes on three laws, which depend for their 

 proof upon observation and induction, 



1, The rays of light are straight lines. 

 J97 L4 



