156 Experimental Philosophy. [Lecture 11. 



than that of the finest painter. I mention the 

 candle as being an obvious luminary ; but if any 

 object whatever is placed at the proper distance 

 from a convex glass, its picture will be, in the 

 same manner, thrown behind, and may be re- 

 ceived upon paper, or any other body, in all its 

 natural proportions and colourings. The nearer 

 the natural object is to the refracting glass, the 

 farther off will this picture be behind it ; be- 

 cause, as was said before, the rays which form 

 it do not then converge or unite, but at a great 

 focal distance. The farther off the natural ob- 

 ject is, the nearer will be the focal distance it 

 makes, and consequently the nearer will be the 

 picture behind the glass ; for, wherever the focus 

 is, there will the perfect picture be. When 

 however the rays come from several objects at 

 a moderate distance, they may be considered as 

 all parallel, and this difference of focus is then 

 imperceptible. 



To put what has been said in other words. 

 As the rays of the sun may be all considered as 

 falling parallel upon every glass of the convex 

 kind, so they must always unite behind it in a 

 focal point. As all the rays flowing from other 

 objects are not always parallel, when placed too 

 near the glass, they separate after refraction, and 

 run off divergent ; when placed at a proper dis- 

 tance, they unite or converge in a focal point, 

 and there imprint a picture, if there is any thing 

 properly placed to receive it, in which the natural 



