178 ON THE SENSES. 



ficult. If the eye of some animal be extracted, and botli tunics, the sclerotic 

 and choroid, be carefully detached from its posterior hemisphere so that the 

 retina may be seen, the delicate image of any bright object presented to the eye 

 may readily be observed on that membrane. An ingenious method has, more- 

 over, been devised, by means of which we are enabled to see in the living eye 

 the image of a candle-flame, for instance, thrown upon the retina. The in- 

 strument by which this is efPected has been called " the eye-mirror," ( Augen- 

 spiegel) ■ to discuss its principle would detain us too long, but its operation is 

 briefly this: the rays of light thrown back from the retina of the observed eye 

 and returning in the same route by which they entered, again unite in the lu- 

 minous point from which they proceeded, whence they are reflected, by the 

 mirror, on the eye of the observer. 



If we observe on the glass tablet of the camera obscura the images of objects 

 situated at different distances from it, we perceive that only the objects which 

 lie at some definite distance throw upon the tablet sharp and distinct images, 

 while the images of objects either nearer or more remote have a confused and 

 faded appearance. If we change the distance between the glass tablet and the 

 lens, or, in other words, shove the lens forwards and backwards, the objects 

 which were at first distinct will become obscure, while the nearer or more remote 

 objects become alternately distinct. In a word, only the images of objects lying 

 at a determinate distance from the lens can be at any one time distinct and well 

 defined ; hence the photographer, in order to obtain an exact likeness of a per- 

 son in front of the camera, must bring the glass tablet or the sensitive collodion 

 plate, introduced in its stead, accurately into the plane on which falls a sliarp 

 image of the object. This proceeding is grounded on the following physical 

 facts : If we imagine a luminous point situated at some given distance before a 

 transparent lens, such as is used in the camera obscura, the rays proceeding 

 from that point and striking the lens 'unite behind the latter at a definite dis- 

 tance ; now, if we bring the objective point nearer to the lens, the point of union 

 of its rays will be changed, and, consequently, the position of its image, which 

 will be thrown farther back beyond the lens ; if, on the contrary, we remove 

 the objective point to a greater distance from the lens, the position of its image 

 will be moved forward and nearer to the lens ; if this objective point lies at an 

 immense distance from the lens, like the sun, the position of its image falls upon 

 what is called the focus or burning point of the lens ; if the objective point, on 

 the other hand, be placed in the anterior focus of the lens, the position of its 

 image will lie at an infinite distance behind the lens — that is to say, beyond the lens 

 the rays proceed in a parallel direction. Let us suppose now three objective points, 

 abc, placed simultaneously before the lens L at definite distances in a right line one 

 after another ; then the corresponding positions of their images a' b' c' will also lie 

 behind the lens in a right line one after the other, but by no means at a distance from 

 one another corresponding with that of the objective points ; the latter, for instance, 

 may be respectively ten, twenty, and thirty feet distant from the lens, the former 

 perhaps only some lines apart. If we suppose that the blank tablet G which in- 

 tercepts the image occupies the place at which the image of the middle object 

 b' is formed this image will be distinctly defined, while the images of the nearer 



Fig. 2. 



