"I may remind you," I proceeded, "that when steam navigation was first mooted, it was confidently asserted that no steamship would ever succeed in crossing the Atlantic Ocean, and I can remember when it was learnedly demonstrated that it would be quite impossible to construct a canal across the Isthmus of Suez! How small the prophets must have felt when the work was accomplished!
"I am afraid it is usual to take a very limited view of all such matters, and we judge of them entirely from what we know ourselves, never looking ahead, as it is considered unscientific to go beyond our own knowledge. Because something may be quite impossible to us, it does not follow that it is impossible to more advanced people.
"Think how many great scientific facts which are quite commonplace at the present time were unknown and undreamed of even so recently as our grandfathers' time! Who then can forecast what may be possible five hundred years, or even a century hence; and who will be bold enough to fix a limit to the possibilities of science! I freely admit I am an optimist in these matters."
"I think, Professor," said John, "that your view is really the more scientific of the two. While it may not be possible accurately to forecast all the facts, intelligent anticipations may logically be formed from a survey of our own past history."
Proceeding, I then remarked, that "Another discovery made at Flagstaff Observatory was that at the ends of certain canals, where they joined the dark areas, were small V-shaped dark markings which Professor Lowell has termed carets. From their occurrence in these positions only, and from his observations of the peculiar and extremely systematic manner in which the canals, especially the double ones, run into the carets, he has concluded that they must serve some special and important purpose.
"We have been told upon high authority that the carets are illusions, and could not possibly be seen, as the planet is so distant from us. But the fact remains that they have frequently been seen and drawn; they always appear the same, and are never seen except in connection with canals which join dark areas. These dark areas, I may say, are believed to be the beds of ancient seas, from which the water has long since departed.
"In connection with all these disputed lines and markings it has often been urged that though they are seen through comparatively small telescopes they are not seen when a very large instrument is used; and it has also been said that observers, knowing what they wished to see, simply imagined they did see it. We have, however, abundant proof that both these arguments are unreliable and incorrect.
"It is a well-known fact that when a faint object has been once seen through a telescope, others are able to see it through a smaller instrument. This was the case with the satellites of Mars, which have been seen with much smaller instruments than that used to discover them.
"The fact that such objects are really seen is proved by the observer marking them on his drawing in their correct position, although they may have moved from the point at which they were originally seen.
"I will give you an illustration of the ease with which it is possible to overlook something that should be clearly visible to you, yet it is not seen by you until your attention is called to it by some one else. Almost every one has had some such experience:—