158 Experimental Philosophy* [Lecture 11. 



of one body, the eye, in fact, is impressed with 

 thousands of radiant points sent out from that 

 body, which being grouped at the bottom of the 

 eye, imprint the picture of the object whence 

 they flow. Each point sends forth its own rays. 



It is upon this principle the camera obscura is 

 constructed. If we take a double convex glass 

 and adapt it so as to fit a hole in the window- 

 shutter of a darkened chamber, so that no light 

 shall come into the room but through the glass ; 

 then let us place a sheet of white paper behind it 

 at the proper distance, we shall thus have a ca- 

 mera obscura ; for a picture of every external ob- 

 ject will pass through the glass, and be painted 

 upon the paper in the most beautiful colours that 

 imagination can conceive, and all the motions of 

 those objects also. It is necessary, in this ex- 

 periment, that the window should not be opposite 

 to the sun ; for then we should see no image but 

 that of his brightness : and yet it is necessary 

 also, that while we make the experiment, the sun 

 should shine and illuminate the objects strongly, 

 which are to paint themselves within. Without 

 this strong illumination, the rays will be sent so 

 feebly from every object, that we shall have but 

 a faint picture, if any at all. 



Painters and architects often make use of a 

 similar contrivance, or portable camera obscura, to 

 take a draught of landscapes or buildings : their 

 glass is fixed in a box, and by means of a mirror, 

 on which the diminished pictures fall, they are 



