Pagina:Scientia - Vol. VII.djvu/268

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260 scientia

same character; is it possible to suppose, if telescopes develop in the future as they have done in the past, that Mr Lowell’s 186 «oases» will continue to present the same uniformity of appearance, any more than the two spots observed by Beer and Mädler?

Further, if we give a novice a small telescope and set him to observe Mars, he will draw the Lacus Solis and the Sinus Sabaeus just exactly as Beer and Mädler did, as two round uniform spots, And the same observer as he gains experience and his instrumental power is increased, will draw those same regions as Dawes and Schiaparelli have shown them. It is no question of planetary change; it is a question of experience and of «seeing».

There is a much simpler explanation of the regularity of the «canals» and «oases» than to suppose that an industrious population of geometers have dug them out or planted them. We know that a telegraph wire seen against a background of bright cloud can be discerned at an amazing distance. For average normal sight, the wire need only subtend a breadth of a second of arc to be thus discerned, and the perception of the wire will be quite unmistakeable. At the same time, it would be quite untrue to say that the perception of the wire was of the nature of defined vision, as will be seen at once when small objects of different form are being experimented with. If instead of a wire of very great length, extending right across the field of view of both eyes, a short line be drawn on a white ground, it will be found that as the length of the line is diminished below a certain point, so its breadth must be increased, and that by the time that the length has been diminished to half a minute of arc the breadth must have been increased to that same amount. The object then, whatever its actual shape, can be just recognised as a small circular spot. The limit for the average observer is indeed a little greater than this, being about 34 seconds of arc.

But even here, though a black spot 34 seconds in diameter, on a white ground, can be perceived, we have not yet attained to defined vision. For, if we place two black spots, each 34 seconds of arc in diameter, near each other, they will not be seen as separated spots, unless they are 4 minutes of arc apart. Nearer than that, they will give the