"But what observations can we make," I asked, "that could not be made from the surface? When I became satisfied from seeing the sun shining through the southern verge, that we had passed into an inner world, I expected with the telescope, to be able to scan every part of the surface, but I found that I was seemingly as far from being able to do so, as when I was in the outer world. Can you explain to me why I cannot turn my glass to the zenith and see the opposite side of the concave?" "There can be but one reason," said MacNair, with a merry twinkle in his eyes. "The gaseous contents of the concave must be opaque to your vision." "Well, well," I said laughing, "I found that out without your assistance, and I am not going to let you dodge the question by a play on words. What I want to know is, why these gaseous contents at the center, are opaque while the air at the surface is not?" "Well I see," said MacNair, "that you are determined to compel me to reveal how little I know. The scientists of the early ages evolved the theory that the center of the concave is a gaseous globe composed of the very lightest materials which they knew by actual experience to be opaque to their vision." "But why," I asked, "is it that this concave sphere does not shut off the light from the sun?" "Because," said MacNair, "this opaque sphere is above our line of vision,--our position on the surface, being twelve degrees below the verges. Besides this, the central opaque sphere is conceived to be flattened at the poles and bulged at the equator, and some have contended that it is also hollow like the earth. But for this opaque sphere our nights would be as light as day by the reflection from the hemisphere above." "I have thought of that," I replied, "and still I have so much wished that the opposite hemisphere could be seen with the telescope." "Well, that is precisely what you will be able to do from this airship," said MacNair. "How so?" I asked. "We certainly cannot rise above the opaque sphere, and if we could, and got a clear view of the opposite hemisphere, that would not be seeing from one side of the concave to the other."