Vision and Optical Glasses. 187 



where the retina is situated ; but if they were 

 suffered to pass on without interruption, would 

 converge in some part behind it. On the con- 

 trary, in figure 69, you see that, from the great 

 convexity of the cornea, the rays are made to 

 converge too soon, and, in effect, the perfect 

 and distinct image is formed in the midst of the 

 vitreous humour, and before it reaches the retina. 



From what you have already learnt of the na- 

 ture of lenses, you will be able to comprehend 

 that the remedy for the former of these defects, 

 that is, where the eye is too flat to cause the rays 

 to converge in their proper place, is a double 

 convex lens, the property of which is to increase 

 the convergency of rays. The focus of this glass, 

 however, must be exactly adapted to the wants of 

 the eye for which it is intended. As therefore 

 the eye grows flatter from age and infirmities, 

 this will explain what is meant by " spectacles 

 for all ages." Where the defect of sight is not 

 great, as in younger persons, spectacles not very 

 convex will suffice; but where the eye is very 

 flat, as in old persons, glasses of a stronger mag- 

 nifying power will be required. 



On the contrary, near sighted eyes (such as 

 fig. 69) being too convex, it is necessary to pre- 

 vent the rays from converging too soon, which 

 can only be done by means of a concave glass, 

 which renders convergent rays less convergent. 

 This glass, however, must also be exactly adapted 



