422 OPTICAL INSTRUMENTS. 



in a dark room, the chalk will be less luminous than the iron. 

 If a piece of stone- ware of black and white pattern (Fig. 324) be 

 heated to redness and viewed in a dark room, the black will shine 

 more brightly than the white, the pattern being reversed as shown 

 in Fig. 325. 



EXERCISES. 



1. Give the best reason you can think of, why the rainbow is a 

 circular arc and not a straight line or of some other shape. 



2. Taking the velocity of light to be 188,000 miles per second and 

 the wave-length for green light to be .00002 of an inch, how many 

 waves per second beat upon the retina of an eye exposed to green light ? 



3. How may spherical and chromatic aberration caused by a lens 

 be corrected ? 



4. Describe Fraunhofer's lines and tell how they may be produced. 

 Why not through a circular orifice ? 



5. Describe in full what is meant by dispersion and the dispersive 

 power of a medium. 



Recapitulation. In this section we have considered 

 the Dispersion of light; the Solar Spectrum 

 and Fraunhofer's Lines ; the Color of bodies ; 

 the Rainbow ; Chromatic Aberration and 

 Achromatic Lenses ; Luminous, Thermal 

 and Actinic Spectra; the Electric Light; the 

 relation between Radiation and Absorption. 



ECTION V. 



OPTICAL INSTRUMENTS.-POLARIZATION. 



656. Photographers' Camera. The photogra- 

 pher's camera is nearly the same as the camera-obscura 

 described in 585. Instead of the darkened room we have 

 a darkened box, BC\ instead of the simple hole in the 

 shutter, we have an achromatic convex lens, placed in a 

 tube at A. 



