THE AUCH OF THE SKY. 215 



CHAPTER XXIV. t 



ILLUSIONS EXPLAINED. 



THERE are few persons whose ideas of the reality, in re- 

 spect to external nature, are not still so far under subjec- 

 tion to the impressions of the senses that they are not 

 easily to be convinced that the arched appearance of the 

 sky is an illusion. 



In conversing on the subject with John a few days after 

 the breakfast with Flippy in the Palais Royal, Lawrence 

 reasoned in this way : 



" The arch in the sky, that looks so much like a reality, 

 seems to come down to the ground at a distance of per- 

 haps four or five miles from us." 



John admitted this, only he had always thought it was 

 farther than five miles to the place where the sky seemed 

 to come down to the ground. 



"It makes no difference," said Lawrence, "what we sup- 

 pose the distance to be. Call it ten miles, if you please. 

 Whatever the distance is, if we go to that place we shall 

 find the sky as high there as it is here. Thus, wherever 

 we are, we have a sky over our heads as high and as arched 

 in one place as in another. If there were any thing real 

 in this arched appearance, the whole surface of the globe 

 would be covered with domes, like inverted cups, cutting 

 each other in every conceivable way. This idea is evi- 

 dently absurd. The truth is, that the dome-like form of 

 the sky is an illusion. It results from certain laws in re- 

 spect to the motion of light, and the effect which is pro- 

 duced upon our sense of vision by rays coming from dif 



