8 EXCUKSIOXS ROUND LONDON. 



which exist between external objects and 



f 



the different organs of our senses. 



I remarked that these relations are of three 

 kinds or species. The first comprehends 

 those of absolute necessity ; the second, 

 those of utility ; and the third, those of plea- 

 sure or satisfaction. 



I denominate the relations of necessity 

 those without which the eye could receive 

 no exercise ; or, in other words, those, 

 without which, its complicated and curious 

 structure would be wholly useless. 



Thus, for example, not only is light ne- 

 cessary to render objects visible, but it is 

 also requisite that our eye should possess its 

 present organisation, in order to perceive 

 the richness and variety of their colors. We 

 are so accustomed to the light of the sun, 

 that it seldom attracts our attention ; and, 

 as we advance in life, we become daily less 

 sensible to its brilliancy. Many individuals, 

 in their progress from the cradle to the tomb, 

 never bestow a single thought on the phe- 

 nomenon 



