464 POPULAR SCIENCE MONTHLY 



ILLUSIONS OF VISION AND THE CANALS OP MAES 



By Professor ANDREW ELLICOTT DOUGLASS 



UNIVERSITY OF ARIZONA. 



rpHAT fascinating mystery, the planet Mars, will again approach 

 -*- the earth this summer. Again the nightly watcher will note 

 the diminishing snow caps at the poles, the dark areas of vegetation, 

 enlarging with the welcome moisture, and, perchance a cloud or two 

 that, lingering over the cold Martian night, is dissipated in the sun- 

 rise heat, revealing thus its character. 



Again also will hundreds of fine dark lines appear, which from 

 their straightness and artificial appearance, seem to attest the exist- 

 ence of highly intelligent beings upon our neighbor. 



It is right and natural that we should first regard these faintest 

 of markings as realities upon the planet. The writer can certify to 

 their apparent genuineness, for he has pictured numbers of them in 

 half a dozen favorable oppositions since 1892. To him they were real 

 until time proved that in the faintest markings astronomers failed of 

 satisfactory agreement. In the larger markings, and even in the larger 

 canals, conflicts of evidence do occur, but are never troublesome. One 

 may confidently say that such realities do exist. But with the very 

 faint canals whose numbers reach occasionally well into the hundreds, 

 discordance reigns supreme, and it is frequently found that different 

 drawings by the same artist antagonize each other across the page. 



Considerations along these lines led the writer to study seriously 

 the origin of these inconsistent faint canals by the methods of experi- 

 mental psychology, and the application of those methods has resulted 

 in a new optical illusion and new adaptations of old and well-known 

 phenomena, all of which apply profoundly to the case in hand. Their 

 description and application follow. 



Halo 



The most important of these phenomena is the halo. 



To observe this, place Fig. 1 at a distance of six to eight feet from 

 the eye and look at it from time to time, taking care to avoid fatigue. 

 Around it will appear a whitish area limited externally by a faint 

 dark line forming a perfect circle, as if traced by a pair of compasses. 

 This external ring or secondary image has a sensible width and appears 

 blackest on its sharp inner edge. When once caught, which is usually 



