two lines, one on each side of the source of light, at a distance of 0″.31 from it, and, since the second maximum has but.017 the intensity of the first, the outlying lines will be but.017 of the brilliancy of the central one.
On Mars we have to consider dark lines on a light field, and little seems to be known of their diffractive effects. There is a disposition to assume that we are here dealing with an inverted diffraction curve. Personally, it seems to the writer that there is no similarity between a bright line, whose light waves produce diffractive effects, and a dark line that emits no light waves. But let us assume that, somehow, the dark line on a light field will produce the same diffractive effects as a light line on a dark field. Then, were the double canals due to this diffraction, they would appear as follows on the planet, when seen in the Lowell refractor. Each and every canal would appear triple, the outer lines would always be separated by 0″.31 from their primary, and be.017 less distinguishable than it. Furthermore, there would be a dark ring around every oasis. No triple canal has ever been observed on Mars, nor has any ring ever been seen around an oasis.
The distance from the first minimum to the second maximum on the diffraction curve measures about ″.08 in the Lowell refractor. Now if the double canals were dark bands of a width of ″.16, then the points of light on the planet, at such a distance from the band that their first minima fell on its edge, would cast the light of their second maxima in the center of these bands, and these maxima, from the points on each side of any band, would overlap.
It is conceivable that such an effect might look something like a double canal, were it not for the fact that the diffracted light from all the other neighboring points of light would swamp and drown any such illusion. Supposing, however, that the double canals were really such dark bands, illuminated in their centers by the second maxima of the fringing light, then the double canals would always appear very nearly 0″.16 apart, which would correspond to about 1°.5 on the planet, when its diameter was 12″. But as the planet approached, since the distances apart of the maxima and minima in the focus of the telescope remain constant, the widths of the bands would no longer fit them, and the effect would be lost. Thus it follows that these bands of uniform width could never appear double, except at one given distance of the planet.
There are certain rules that the double canals should observe if they were due to diffraction, but they follow none of these. They should (since the size of the rings of diffraction remain constant through a given aperture) appear nearer together, in degrees on the planet, as Mars approaches; instead of which they remain the same size. They should (as diffractive effects vary in size inversely as the radius of the objective). as the objective is diaphragmed down, appear