[[Categoria:Pagine che usano RigaIntestazione|Scientia - Vol. VII.djvu{{padleft:271|3|0]]over, 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 finally examined by the eye. The granular structure of the plate 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