B(5 SENSE OF SIGHT. 



S. In order to see an object, the rays of light which emanate 

 from it, or which are reflected by it, must reach to the bottom of 

 the eye. For this reason, an opaque body placed between the 

 eye, and the object at which we look, renders the latter invisible. 



9. The surfaces of opaque bodies do not always reflect back 

 the light the same as they receive it. As we have said, there are 

 some which absorb all the rays; such bodies are called Wack. 

 Bodies that reflect all the rays, or nearly all, are white, but tnose 

 which decompose them, are coloured 



10. Colour is not inherent in bodies ; it depends upon the man- 

 ner in which they decompose the light, and the kind of luminous 

 ray that the coloured body can reflect. Each ordinary ray of 

 light, though it appears colourless to us, is composed of seven dif- 

 ferently coloured rays: there is a very simple mode of being con- 

 vinced of this fact; if we receive a bundle of luminous rays, 

 which have passed through a glass prism, upon a sheet of paper, 

 instead of producing a white image, it will form an oblong image, 

 in which we distinguish the following seven colours, namely : 

 Red, Orange, Yellow, Green, Blue, Indigo. Violet Now, objects 

 appear to us white, when they reflect the light, without decompo- 

 sing it, and coloured in this or that manner, when they decom- 

 pose it like the prism, and absorb some rays and reflect others. 



11. In passing through transparent bodies, rays of light some- 

 times continue to follow their primitive direction : but on other 

 occasions, they change their direction, and approach towards, or 

 diverge from each other. For example, when a straight stick is 

 plunged, half of its length, obliquely into water, it seems as if it were 

 broken ; and it is by acting in this way upon light, that the con- 

 cave or convex glasses of spectacles, enlarge or diminish the 

 images of bodies. This deviation of light is called re.frtictin. 



12. In order to see a body, the rays of light which part from 

 it, must reach the bottom of the eye, and there paint an image of 

 the object; the impression thus produced, is received by a par- 

 ticular nerve, and by it transmitted to the brain which receives 

 the sensation. 



13. The apparatus of sight is composed: 1st. of the organ of 

 vision, which consists of the globe of the eye and its nerve; 2nd. 



8. What ic necessary to cnnble us to perceive n object ? 



9. Do all bodies reflect light ? What is the colour of those bodies whioh 

 bsorb all the rays ? Whut is the colour of those bodies which reflect all the 

 rays ? What is the colour of (hose bodies which refract the rays? 



10. Upon what does the colour of bodies depend? 

 11 What is refraction? 



12. In what manner is the image of an object conveyed to the brain ? 



13. Of what parts is the apparatus of sight composed ? 



