80 THE EYE. 



gent upon the crystalline humour, and consequently includej 

 a greater angle, and thus the object is magnified. In objects 

 placed at such distances as we are used to, we know, by, 

 experience, how much an increase of distance will diminish^ 

 their apparent magnitude, and we instantly suppose them of| 

 the size they would appear if they were less remote ; but^ 

 this can only be done, where we are well acquainted withi 

 the real magnitude of the object ; in all other cases w® 

 judge of magnitudes by the angle under which the object 

 appears at tlie known, or supposed distance ; that is, we infer 

 the real magnitude from the apparent magnitude in compari-j 

 son with the distance of the object. Sight, therefore, does no^ 

 represent extension such as it is in itself; it often deceiveai 

 us both in regard to the size and the distance of objectsj," 

 and we should be led into continual errors if experience di(| 

 not set us fight. This is rendered strikingly manifest^ 

 from the case of a young man who was blind fiom his mri 

 fancy, and who recovered his sight at the age of fourteen,^ 

 by the operation of couching. At first he had no idea either' 

 of the size or distance of objects, but imagined that uvery^ 

 thing he saw touched his eyes ; and it was not till aften 

 having repeatedly felt them and walked from one object toj 

 another, that he acquired an idea of their respective dimen-*! 

 sions, their relative situations, and their distances. 



Questions. — 1. What is the external covering of the eye called .'—, 

 Describe it. 2. Describe the cornea. 3. The choroid. 4. The pupil. 51 

 The iris. 6. What is said in order to illustrate the contraction and dila«^ 

 tation of the iris .^ 7. Of what use is the black liquor in the choroid .'j 

 8. Describe the three humours of the aye. 9. Of what does the retfna! 

 consist, and what is its use ? 10. How is the image on the retina ren- 1 

 dered distinct ? 11. How does it appear that the image on the retina- 

 will be inverted ? 12. Having two eyes, why do we not see objects^ 

 double ^ 13. Why does a distant object appear smaller than one that] 

 is near ? 14. How do we judge of the real magnitudes of objects ;\ 

 15. What case is related to show that experience is necessary to correct"; 

 the errors of sight ? IG. Look at fig. 28. and describe the eye.j 

 [Note. Let the instructor explain to his pupils how objects of equals 

 magnitudes appear under a greater angle when near, than when at aj 

 distance.} I 



