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|263||THE “ CANALS „ OF MARS|
is monocular; our unassisted sight is binocular, and therefore the more efficient in the recognition of minute detail. Telescopic vision, therefore, increases the tendency to display minute markings under the two « economic » forms of straight lines and circular spots.
When we come to photographs, the process is carried to a third stage. The image is formed by the telescope and is received on a plate essentially granular in structure, and is Anally examined by the eye. The granular structure of the pi ate acts as a third factor in reducing irregularities and simplifying details; a third factor in producing the two simplest types of form, the straight line and the circular spot.
The above conclusions were reached by me in the year 1891, whilst reviewing a paper by M. Camille Flammarion in which the latter gave a number of instances in which sunspots were seen with the naked eye. I was surprised to note how often groups of spots of relatively small total area had been seen, whilst others very considerably larger had escaped detection. I therefore for some time watched the sun on every available occasion without any optical assistance except that of an ordinary dark glass, and found that, I could often recognise the presence of a straggling group of small unimportant spots as a short « canal », when a single spot of considerably greater total area was quite invisible. These observations and the experiments which followed them, led me to the conclusion that in all probability the « canals » of Mars were simply the summation of a complexity of detail far too minute to be separately discerned. (« Knowledge », 1894, pp. 249-252, and 1895, p. 58).
A little later, in his work « Marte nel 1896-97 », Dr Cerulli independently arrived at the same conclusion and wrote « These lines are formed by the eye.... which utilises.... the dark elements which it finds along certain directions »; that « a large number of these elements forms a broad band »; and that « a smaller number of them gives rise to a narrow line ». Also « the marvellous appearance of the lines in question has its origin, not in the reality of the thing, but in the inability of the present telescope to show faithfully such a reality ». Then, pushing the theory further, Dr Cerulli discovered the remarkable fact that an opera glass reveals « canals » on the Moon, while in a recent letter to M. Antoniadi, he showed