LECT. XX. CAMERA OBSCURA. 361 



an impression on our eyes. In this lecture we shall examine 

 the way in which the rays of light, emanating from external 

 objects, reach the retina and excite it. 



If, in the dark, you press or strike the eye, a vivid, but 

 indistinct luminous sensation is produced. If the pressure 

 be made by means of a small body on a limited extent of 

 the eye, the sensation will be equally limited, and you will 

 be conscious of its limit; that is the degree of pressure ex- 

 ercised upon the compressed point. 



Optical Apparatus required to form Images. If the sur- 

 face of the retina were presented naked to the luminous ob- 

 ject, without any optical apparatus being placed in front of 

 it, it is obvious that every point of it would be stimulated 

 at the same time, by all the rays which proceed from the 

 object in every direction. And if thousands of those rays, 

 with their divers colours, simultaneously presented them- 

 selves before the eyes, they would all at the same time give 

 rise to the impression of their light upon the retina, which, 

 however, would not have an exact and definite sensation of 

 any of them. 



The problem to solve to obtain vision, consisted, there- 

 fore, in placing before the retina an optical apparatus by 

 which the rays of light, emanating from the various parts of 

 an object, should separately reach distinct parts of the re- 

 tina, and in a given order. 



1. The Camera Obscura. The camera obscura is the 

 most simple apparatus which we possess for obtaining these 

 effects. Fancy a diaphragm with a small aperture in its 

 centre, placed before the retina : the rays proceeding from 

 one extremity of the object, passing through the aperture, 

 will excite a certain point of the retina ; and the same will 

 take place with all the other parts of the same object, which 

 will excite a corresponding number of parts of the retina. 

 The smaller the aperture, the more defined will be the 



