Page:Popular Science Monthly Volume 71.djvu/283

Rh detail. Indeed it often seems to be the reverse; as if an eye, sensitive to light, were not as acute for form. Now it is well known that no two objects can be separated by the human eye that do not fall on more than one cone in the fovea, or central pit of the retina. So may it not be that an eye, very sensitive to light, has unusually large cones in the fovea, while one acute for form has small ones?

There seems to be a great reluctance to accept the finer markings on Mars as established facts, and their objective reality has been questioned by all kinds of doubts and theories by all sorts of men. It might be well if some people, who explain away the markings on the supposition that they are optical illusions, would take the trouble to follow up their theories, see where they lead to, and work out what the appearance of the markings would be if due to the causes they suggest.

A Professor Douglass, of Arizona, has lately suggested that the canals are nothing but the black rays that can be seen radiating from a black spot, on a light ground, when looked at with a small screen placed in front of the pupil of the eye, so that the light enters only around the edges. According to Professor Douglass, the black oases are the only real things in the canal system, while the canals themselves have no tangible existence, and are nothing more than these black rays issuing from the dark spots.

In the first place, no eyepieces, constructed on any such principles as Professor Douglass uses to see these rays, are known at Flagstaff. Furthermore, the oases are more difficult objects to see than the canals. The latter are often visible when the former are not. It would seem that even Professor Douglass should find it hard to admit that, at such times, the canals are visible radiations from invisible spots.

But let us see what the canals would look like if actually due to this cause. These radiations are due to irregularities in the crystalline lens, and are constant for an adult, but vary with each individual. So that any one, looking at the planet, would see an exactly similar set of radiations issuing from each oasis. The planet would be covered with a quantity of black spots, all with similar radiations, and all absolutely independent of each other. For no two radiations from different spots would ever, except by the rarest chance, run into each other to form a straight unbroken line, connecting the two spots. The radiations would also be entirely different for each individual. As one of the most striking features of Martian detail is the manner in which the canals connect and interlace the oases, further comment seems unnecessary.

It has also been suggested that the so-called canals may not be lines at all, but merely a disconnected string of broken markings, a sort of irregular dotted line.

A series of observations at Flagstaff, conducted by several individuals, has shown that the eye is extremely sensitive to a break in a line.