INTRODUCTION TO OPTICS. 



Ixxxi 



not till after having repeatedly felt them, and walked from one object to 

 another, that he acquired an idea of their respective dimensions, their 

 relative situations, and their distances. 



Since an image is formed on the retina of each of our eyes, it would 

 seem that we ought to see objects double. In fact, however, we do not; 

 and perhaps the best solution which has been offered of the difficulty is 

 this, that the action of the rays on the optic nerve of each eye is so perfectly 

 similar, that they produce but a single sensation: the mind, therefore, 

 receives the same idea from the retina of both eyes, and conceives the 

 object to be single. It is, however, safer to treat the fact as one estab- 

 lished by experience, but not admitting of any satisfactory explanation ; 

 for the manner in which external objects act upon the mind admits of 

 no direct observation, and all theories respecting it can therefore rest on 

 no sound foundation. Persons afflicted with a disease in one eye, which 

 prevents the rays of light from affecting it in the same manner as the 

 other, frequently see double. 



The image of an object in a looking-glass is not inverted, because 

 the rays do not enter the mirror by a small aperture, and cross each other, 

 as they do at the orifice of a camera obscura, or the pupil of the eye. 



When a man views himself in a mirror, the rays from his eyes fait 

 perpendicularly upon it, and are reflected in the same line ; they proceed, 

 therefore, as if they had come from a point behind the glass, and the 

 .same effect is produced, as if they proceeded from an image of the object 

 described behind the glass, and situated there in the same manner as the 

 object before it. This is not the case only with respect to rays falling 

 perpendicularly on the glass, but with all others. Thus in Jig. 10, a 

 ray proceeding from the point C to D is reflected to A, and arrives there 

 in the same manner as if it had proceeded from E, a point behind the 

 glass, at the same distance from it as C is in front of it. 



A man may see himself at full length in a mirror which is not more 

 than half his height (Jig. 10). The ray of light, A B, from his eye which 

 falls perpendicularly on the mirror, B D, will be reflected back in the 

 same line ; but a ray, C D, from his feet, which falls obliquely on the 

 mirror, will be reflected in the line D A ; and since we view objects in 

 the direction of the reflected rays, which reach the eye, and the image 

 appears at the same distance behind the mirror as the object is before it, 

 we must continue the line A D to E, and the line A B to F, at the termi- 



Fig. 10. 



Fig. 11. 



