SENSE OF SIGHT. 



deafness being the necessary consequence of their loss; they are 

 absent in a great many animals. 



49. For example, birds have not the pavilion of the ear; iep- 

 tiles are destitute of the pavilion, and the auditory canal ; in fish 

 all parts of the middle ear, or tympanum, are wanting, and in 

 other animals, such as the craw-fish, the apparatus of hearing 

 consists only of a small vesicle similar to the vestibule. 



LESSOR IX. 



FUNCTIONS OP RELATFOS. Ssnse of sight Light Apparatus 

 of vision Eye-brows Eye-lids Lachrymal apparatus Mus- 

 cles of the eye Structure of the eye Use of different parts oj 

 the eye Voice. 



OF THE SIGHT. 



1. Sight is the sense by which we perceive the form, colour 

 volume, and position of objects that surround us. 



2. This sense, which Buffbn called " distant touch," is exer- 

 cised at a distance, through the medium of light. 



3 To comprehend the mechanism of sight, it is not sufficient 

 to know the structure of the eye ; we must also be familiar with 

 some of the properties of light, the study of which subject be- 

 longs to that branch of science called Optics. 



4. Light is a fluid which tills space and illuminates the earth. 

 It emanates from luminous bodies, such as the sun, the fixed stars, 

 and substances in combustion, and diffuses itself afar with incon- 

 ceivable rapidity. 



5. In proportion as the rays become distant from the body 

 from which they emanate, they diverge one from the other, and 

 for this reason bodies are better lighted, the nearer they are to 

 the illuminating body. 



6. When light meets with a body, it either passes through it, 

 or is reflected from it, or it may be absorbed. 



7. Those bodies which permit light to pass through them are 

 called transparent ; those which oppose its passage, are called 

 opaque,. 



49. Is the organ of hearing the same in all its parts in all animals ? 



1. What is sight? 



2. By what me;ins is the sense of sight exercised ? 



3. Is a knowledge of the structure of the eye sufficient, in itself to teacfa 

 Us the mechanism of sight ? 



4. What is light? What are the sources of light? 



5. Why are bodies better lighted when near the illuminating body'/ 

 G. When li^hl meets with a body, what takes place / 



1 When are bodies transparent? When are bodies 



or 



