356 WELLS'S NATURAL PHILOSOPHY. 



ible to ordinary sight. At four miles off, a man becomes thus invisible, and 

 a pin-head near by will hide a house on a distant hill.* 



What do ^e "^l^- When we say we see an object, we 

 ?iy* wr^'see In dean that the mind is taking cognizance of a 

 object? picture or image of the object formed on the 



retina. The manner in which the sensation is conveyed 

 by the optic nerve to the brain, and a knowledge of tho 

 external object imparted to the mind, is entirely un- 

 known. 



^ ^. As the picture, or imaoro on the retina, is formed on a corn- 



Does the sense ■ ■, n r o 



of sight give paratively flat surface, the sense of sight can not of itself af- 



c^'uon'^l^^form ^^^^ ^°^ immediate perception of the distance, size, or position 

 size, position, of external objects. This knowledge we gain by experience 

 * '^' derived from continued observation, and from tlie other senses. 



A young child has no conception of distance, and grasps at the moon as if 

 it were an object immediately within its reach. Persons born blind and re- 

 stored to sight by surgical operations, although able to see distinctly, can not 

 properly comprehend any object or prospect before them. " I see men as 

 trees walking," said the man bom blind when restored to sight. Individuals 

 thus situated acquire the correct sense of vision only by degrees, like infants . 

 and it is by experience tliat they learn to walk about among the objects 

 around them, without the continual apprehension of striking themselves 

 against every thing they behold. 



What is Per- Perspective is the name given to that science 

 spective? which tcachcs how to draw on a jilane surface 

 true pictures of objects as they appear to the eye from any 

 distance and in any position. 



The skill of the artist consists in rightly applying tlie laws and principles 

 of perspective ; and a picture is perfect to the extent in which it agrees with 

 our experience of the objects it represents. 



714. Many optical and mental delusions are occasioned 

 in estimating the size, figure, and position of objects, by 



• " The smallest particle of a white substance distinguishable by the naked ere upon a 

 black ground, or of a black substance upon a white ground, is about the l-400th of an 

 Inch square. It is possible, by the closest attention, and by the most f worable directioa 

 of light, to recognize particles that are only l-540th of an inch square, but without any 

 sharpness or certainty. But particles which strongly reflect light may be seen when not 

 half the size of the least of the foregoing: thus, gold dust of the fineness of 1-1 125th of an 

 inch may be discerned by the naked eye in common daylight. When particles that can 

 not be distinguished by themselves with the naked eye are placed in a row, they become 

 visible ; and hence the delicacy of vision is greater for lines than for single particles. 

 Thus, opaque threads of no more than l-490(1th of an inch across, or about half the diam- 

 eter of the silkworm's fiber, may be discerned wiLh the naked eye when they are held 

 toward the light." — Dr. Cai-pentcr. 



