What may be seen with a microscope?
Thousands of things too small to be seen by us unless they are magnified.
What kind of spectacles do near-sighted people use?
Near-sighted people use glasses, thickest at the edge and thinnest at the centre.
What kind of spectacles do we use as we grow old?
As we grow old we use glasses, thinnest at the edge and thickest at the centre.
Of what use are spectacles?
When the vision is defective, spectacles enable us to see objects more clearly.
How should spectacles be made?
They should be so made as to render objects distinct, but neither to magnify nor diminish them.