SHORT SIGHT WEAK SIGHT. 



It sometimes happens that the humours, like coloured glass or 

 coloured liquids, transmit only light of a particular colour. In 

 that case the image on the retina is falsely coloured, the false 

 colours depending on the colour of the humours. 



For these several classes of defects spectacles are wholly ineffi- 

 cacious. 



6. When the humours are perfectly transparent and free from 

 colour, the picture which they would produce may fall not imme- 

 diately upon the retina, hut at a distance more or less considerable 

 before or behind it. In that case the effect produced upon the 

 retina will be a picture more or less confused and indistinct. It 

 will be so much the more indistinct as the place where the dis- 

 tinct picture would be formed is more distant from the retina. 



If the place of the distinct picture be before the retina, the 

 defect is owing to the eye having too great refracting power upon 

 the rays of light. If it be behind the retina, it is owing to the 

 refracting power being too feeble. 



The former is called SHOKT SIGHT, and the latter LONG SIGHT, 



Or WEAK SIGHT. 



7. For these defects of vision, which are by far the most 

 common, spectacles supply a perfect remedy. 



They accomplish this by the effect they are capable of producing 

 upon the place of the picture. If the eyes be weak-sighted, and 

 consequently the picture is formed behind the retina, spectacles 

 are applied which have the effect of bringing forward the picture 

 to the retina. If they be short-sighted, so that the picture is 

 formed before the retina, spectacles are applied which have the 

 effect of throwing it back to the retina. 



8. Spectacles consist of circular discs of glass called lenses, 

 the surfaces of which are brought by grinding and polishing to a 

 convex or concave form. 



9. If a convex lens of this kind be placed before the eye, it will 

 have the effect of bringing forward the picture formed within the 

 eye. A concave lens, placed in the same manner, will have the 

 contrary effect of throwing it backward. 



It will be easy for any person to convince themselves that such 

 glasses have the properties here described. 



Let a convex disc of glass or lens, G G, fig. 1, be placed before a 

 candle, c, and let a white paper screen be placed behind G G, and 

 moved towards and from it until a position is found, such as s s, 

 in which a distinct inverted picture of the candle will be seen 

 upon it. If the screen^be now moved to s' s', a little nearer to G G, 

 so that the place of the distinct picture shall be behind it, an 

 indistinct picture of the candle will be seen upon the screen. 



In' this case the lens G G may be imagined to represent the eye of 

 o2 195 



