270 OPTICS. 



Microscopes. 



To determine the magnifying power of glasses em- 

 ployed in single microscopes. 



If the focus of a convex lens be at one inch, and 

 the natural sight at eight inches, which is the com- 

 mon standard, an object may be seen through that 

 line at one inch distant from the eye, and will appear 

 in its diameter eight times larger than to the naked 

 eye. 



But as the object is magnified every way equally, 

 in length as well as in breadth, we must square this 

 diameter to know how much it in reality appears en- 

 larged ; we shall then find that its superficies is mag- 

 nified 64 times. 



A small globe of glass, or of any transparent sub- 

 stance, makes a microscope. 



Invisible Rays. 



Sir William Herschel discovered invisible rays be- 

 yond the red extremity of the spectrum, which have 

 the power of heating, but not of illuminating, objects. 

 This led to the subsequent discovery of the chemical 

 or de-oxidating rays, at the opposite side of the spec- 

 trum. 



The colour of white is a mixture of all the other 

 coloured rays. 



Magnifying Power. 



The magnifying power of reflecting telescopes may 

 be increased merely by varying the distance between 

 the eye-piece and the great speculum, and then pro- 

 ducing distinct vision by the new adjustment of the 

 small cnirror. 



Strength of Sight. 



There is reason to believe that a good eye in the 

 prime of life, and unaided by glasses, may see an 



