ILLUSIONS OF VISION 473 



rays. At last when they did not show agreement among themselves 

 1 concluded they must be illusions. This was verified by specific trial, 

 proving that such lines appear on perfectly blank areas. The rays 

 so observed are sometimes double. 



Application of these Phenomena 



Against the obstacles of bad atmosphere, minuteness of detail and 

 faintness, the observer has to wage a hard fight, and it is a matter of 

 congratulation that he sees such faint canal-like marks on the very 

 limit of vision. With full records the public may then discuss the 

 interpretation. 



The ray illusion is to me a very satisfactory explanation of many 

 faint canals radiating from those small spots on Mars, called ' lakes ' 

 or ' oases.' The only objective reality in such cases is the spot from 

 which they start. The reader will notice that rays on opposite sides 

 of a star are usually in line. So when two lakes or oases lie along 

 such a line they will appear connected by a canal. Nor do the oases 

 need to be very close together. A ray 16' long to the naked eye ap- 

 pears 4" long on a planet magnified 240 diameters. With the planet 

 Mars 16" in diameter the ray then extends one fourth across it. It 

 appears like a canal over one thousand miles long. 



I believe the industrious observer has found and will find it difficult 

 to avoid instinctively placing his head in a position favorable to pro- 

 ducing combinations of this kind. After he has laboriously memorized 

 the leading details, so that he may recognize what he sees, when, for 

 an instant, Heaven vouchsafes him a brief view, he naturally has a 

 powerful inclination always to observe in the same posture, for he 

 finds that with a slight movement of his head his structure of fainter 

 canals is liable to disorganization. This insistence upon the same 

 attitude is at once understood when we consider a larger part of the 

 faint canals to be due to rays in the eye. 



We have here the medicine to prevent this disease in the future. 

 Let the observer constantly vary the position of the head. As soon 

 as the seeing becomes sufficiently good to reveal fine detail, let the 

 movement of the head begin. A rotation through an arc of twenty 

 or thirty degrees ought to be large enough to test thoroughly any 

 fancied combination of canals. Drawings carefully made in this way 

 will have one source of error eliminated. 



The halo with its light area and secondary image accounts for 

 details which have no objective reality, such as bright limbs of definite 

 width, canals paralleling the limb or dark areas, numerous light mar- 

 gins along dark areas and light areas in the midst of dark — abundantly 

 exemplified in Schiaparelli's map of 1881-2. 



When a ribbon-like mark has sufficient width, it must appear 



