IN PHT81C8. 223 



59. Why can we not see the fence-posts when we are 

 riding rapidly ? 



There is not time for the rays of light to produce a 

 distinct impression on the retina. 



(See Physics, p. 177, line 13.) 



60. Ought a red flower to be placed in a bouquet by an 

 orange one ? A pink or blue with a violet one ? 



(See Physics, p. 167.) 



These are not complementary colors, and so weaken 

 rather than strengthen each other. 



6 1. Why are the clouds white while the clear sky is blue 1 } 



Prof. Tyndall has shown that the larger particles of 

 vapor scatter light of all colors, /. e., white light ; while 

 the smallest particles, only the blue rays. In accordance 

 with this fact the clouds are white and the sky is blue. 

 If the air were absolutely pure and free from all foreign 

 matter, it is thought that the azure of the sky would not 

 be seen and the heavens would appear black : the illu- 

 mination of objects would be strong and glaring on one 

 side, and on the opposite side the shadows would be 

 deep and unrelieved by the diffused , light to which we 

 are accustomed. The minute particles of vapor in the 

 air serve to scatter the direct rays of the sun and to 

 turn them around corners and into places not in the 

 direct line of the sunlight. 



(See a full and interesting discussion in Tyndall's Lecture on Light, page 

 152, et seq.) 



62. Why does skim-milk look blue and new milk white? 



The fatty globules of the new milk reflect all the 

 colors of the spectrum to the eye ; but when deprived 



