THE PLANET MARS 131 



maybe taken, which is the apparent enlargement of a bright disc 

 when seen against a dark background. By trial of the different 

 contrast effects to which this phenomenon is due, its laws may 

 be determined and its effect eliminated from observations which 

 it might otherwise vitiate. 



An instance of the deceptive illusion is the often-quoted 

 power of the eye of integrating minute markings too small to 

 be severally visible. On looking at a mass of small specks too 

 small to be seen clearly apart, the eye has a strong tendency to 

 accept the specious appearance of these as lines and they cannot 

 be distinguished from realities except by the closest scrutiny 

 Happily this illusion is only possible under critical circum- 

 stances of distance on the narrow borderland between seeing 

 the dots as they are and not seeing any trace of them. 



Now the lines which skilled observers have perceived on 

 Mars have been seen under many varied circumstances of 

 distance, illumination and instrument. It seems therefore 

 impossible that they can be due to this form of illusion. Also 

 it is certain that though a series of dots may masquerade as 

 lines, the converse action is inconceivable. Since also dots and 

 lines are visible on Mars at the same time — oases and canals — 

 the assumption of the reality of both seems warranted. 



There is another illusion to which the double canals have 

 been, I think erroneously, assigned, namely double vision. 

 Why double vision should be specified I know not, for multiple 

 vision is equally possible. We all know that by imperfectly 

 focussing an object we may, under certain conditions, see it 

 double and if strong contrast occurs we may in the same way 

 induce multiple vision. 



Now on Mars are many double canals but illusion suggests 

 that the most conspicuous should be double or multiple. On Mars 

 I know of many cases of faint canals which are double and 

 conspicuous ones that are single but none which are multiple. 

 The canals which appear double appear so from some cause on 

 the planet and not in the eye. They are alike indifferent to and 

 inexplicable by any illusion of the observer's eye and the 

 individuality of the behaviour quite definitely shows. It is 

 the failure to explain the Martian markings as the results of 

 illusion that assures us of their reality. 



In this preliminary account I have but summarised the 

 methods and means, the illusions and difficulties which beset 



