94 A CENTURY'S PROGRESS IN ASTRONOMY. 



popularity, and had to give way to a more 

 popular theory, the " illusion " hypothesis, put 

 forward by the Italian astronomer Cerulli, and 

 supported by Newcomb and Maunder. On the 

 basis of the illusion theory, Newcomb explains 

 that the " canaliform " appearance "is not to be 

 regarded as a pure illusion on the one hand or 

 an exact representation of objects on the other. 

 It grows out of the spontaneous action of the 

 eye in shaping slight and irregular combinations 

 of light and shade, too minute to be separately 

 made out into regular forms." Experiments 

 were made by Maunder in 1902, and the results 

 pointed to the truth of the theory that the 

 canals were really illusions. But the studies of 

 Lowell at the oppositions of 1903 and 1905 have 

 seriously weakened the hypothesis of Cerulli and 

 Maunder, and strongly confirm the theory of the 

 artificial origin of the canals. In 1903 Lowell was 

 enabled, from a study of the development of the 

 canals, to show the probability of their artificial 

 nature, and his study of the double canals showed 

 a distinct plan in their distribution. Finally, 

 on May 11, 1905, several photographs of Mars 

 were secured at the Lowell Observatory, on 

 which the canals appeared, not as dots of light 

 and shade, as on the illusion theory, but as 

 straight dark lines. This goes far to prove the 



