] Q RADIATION. 



that an image is formed upon the retina of the eye so as 

 to produce vision. 



If you examine one of the glasses of a pair of spectacles 

 such as are used by elderly persons, and sometimes, indeed, 

 by persons who are still young, but not near-sighted, you 

 will see that the glass is thicker in the middle than at the 

 edges. Such a glass is called a convex lens. If, now, in 

 the evening, you remove or extinguish all the lights in the 

 room but one, and put that light at one side, or in one cor- 

 ner, and then proceed to the opposite corner, or into the 

 darkest part of the room, and there hold a small piece of 

 white paper against the wall, and one of the glasses of the 

 spectacles between it and the light at the proper distance, 

 you will find that an image of the candle, inverted, will bo 

 formed upon the paper or card. The image may be small, 

 but if the experiment is carefully performed it will be beau- 

 tifully distinct and clear. The lens collects and concen- 

 trates the light, and forms an image of the candle upon the 

 paper or the card, which serves as a screen to receive it. 

 Of course this experiment can be performed on a larger 

 scale, and in a much more satisfactory manner, with a 

 proper lens and other convenient apparatus, as shown in 

 the following engraving. 



Now in the eye there is just such a lens and just such a 

 screen that is, just such in respect to function. The lens 

 is in the front part of the eye, and the screen, which is 

 called the retina, is in the back part ; and it is by means 

 of this image on the retina that the picture of the outward 

 object is conveyed to the mind. 



Now when it is said that the whole of the illuminated 

 sphere surrounding a source of light as described is, in a 

 certain seme, filled with light, the meaning is that there is 

 no part of the whole space where an opening no larger 

 than the pupil of the human eye will not take in enough to 



