44 ANSWERS TO PRACTICAL QUESTIONS 



The property of absorbing energy of special wave length, 

 and reflecting that of another wave length which we perceive 

 as color, is a physical property of the object. The perception 

 of this, as of sound, or of temperature, is finally a mental act. 



18. IVTiy is lather opaque, while air and a solution of 

 soap are each transparent ? 



By repeated reflections and refractions in passing through 

 the mass of lather, no ray can pass through in a straight line. 

 Transparency is hence destroyed. 



19. Why does it ivhiten molasses candy to pull it ? 



The viscous mass, by repeated pulling, becomes fibrous. The 

 fibers at the surface reflect light more regularly, and hence the 

 candy appears more nearly white. 



20. Why does plastering become lighter in color as it 

 dries ? 



Because, as the water evaporates, the mortar transmits less 

 light, and reflects more light to the eye. 



21. Why does the photographer use a lamp ivith a 

 chimney of red glass in the " dark room " ? 



Because this glass transmits only the longer waves of light, 

 while chemical effect is produced chiefly by the shorter waves. 



22. Is the common division of colors into " cold " and 

 " warm " verified in philosophy ? 



Yes ; red contains more heat than violet. 



23. Why is the image on the camera, Fig. 177, in- 

 verted ? 



The rays cross each other at the focus of the double convex 

 lens. 



24. Wliy is the second image seen in the mirror 9 

 14O, brighter than the first? 



The first is formed by reflection from the glass, and the 

 second from the mercury. As the latter is a better reflector, 

 the second image will be brighter. Each image after that will 

 be weakened by the repeated reflection. 



