"There can be no doubt that we have solved the problem of life below the water, and whether this be a return to the habits of a remote ancestor, or a totally new faculty we have acquired, is of much less importance than the fact itself.
"It is believed that the first great requisite for our subaqueous life was the invention of a corrective of that defect of vision produced by the contact of the eye with the water. The cause of this defective vision is the optical structure of the eye itself, the refractive power of whose humours differs but slightly from that of water; consequently, when it is immersed in water, the rays of light are not deflected sufficiently to allow the images of external objects, to be accurately focussed on the retina; hence only a very imperfect vision is possible for the unaided eye below the water.
"Our opticians discovered that perfect vision could be restored under water by means of a lens of considerable power. Spectacles were constructed with such lenses, and the first step was made towards rendering life below water practicable.
"At first, we, like yourselves, constructed our lenses of solid glass; but as the refractive power of glass is not very much greater than that of water, there was an enormous difference between the power of a lens used in air and in water. Hence our opticians early adopted the plan of making lenses of all descriptions for use below water, of air. Air being the less refractive medium, our air lenses are of precisely the opposite shape to that of your glass lenses. Our ordinary method of making these air lenses is to have sections of thin glass globes of certain diameters fixed in a ring of metal or bone, with their concavities looking outwards. They thus enclose a concave lens-