Q. What is the use of double concave spectacle glasses?
A. Near-sighted spectacles cast the reflection further back; and the image (being thrown upon the ret’ina) becomes visible.
Q. Why are old people far-sighted?
A. Because the humours of their eyes are dried up by age, and the cor’nea sinks in, or becomes flattened.
Q. Why does the flattening of the cor’nea prevent persons seeing objects which are near?
A. As the cor’nea is too flat, the image of any near object is formed behind the ret’ina of the eye, and is not seen at all.
The reflection is made at DE, instead of at ABC (the retina).
Q. What sort of glasses do old people wear?
A. As their cor’nea is not sufficiently convex, they must use double convex glasses, to enable them to see objects near at hand.