i 3 o SCIENCE PROGRESS 



have arisen from the extreme difficulty of drawing the delicate 

 detail that is seen. We have only to look at various drawings 

 by different observers to be assured of this. Comparing the 

 drawings, it is difficult to believe them to be bona fide attempts 

 to portray the same object. 



Lowell tells me that after twenty years of practice in this 

 particular work, he is quite unable to draw the canals of 

 Mars as they appear in the telescope. His practised hand 

 cannot trace lines on paper fine enough or straight enough 

 to represent them. It is therefore natural that the attempts 

 of less experienced observers should be but caricatures of the 

 planet which they strive to represent. It is, however, a relief 

 that the drawings made independently at Flagstaff do resemble 

 one another and the planet very closely, thus affording internal 

 evidence both of the reality of the features seen and the 

 accuracy of the representations. 



Turning now to the method by which detail is detected, 

 we find that the process, unlike the announcement of the 

 discovery, is not a sudden one. Let us follow the observer 

 to the dome and trace his method. Armed with a suitable 

 dark glass and an appropriate aperture, as explained earlier, 

 he watches the planet carefully. Suddenly he is startled by 

 the appearance of some previously unknown marking which 

 flashes into sight but for a moment and is gone, leaving only a 

 vague impression of something being there. The hint so 

 obtained must be noted, for perhaps, later on, another and 

 another glimpse may be obtained which by their cumulative 

 effects assure us of the reality of the new feature. 



This is the manner in which all the canals have been 

 discovered and just as accumulated observations establish 

 their numbers, so accumulated hints attest the existence of the 

 fainter markings, until a moment of perfect seeing shows them 

 in all their beauty with the fineness and fixity of a steel 

 engraving. 



At first sight their elusiveness suggests an illusion, which 

 accordingly claims our attention next. Optical illusions may 

 be divided into two classes — those which are self-confessed and 

 obvious ; and those specious appearances of reality which may 

 deceive all but the most penetrating analysis. 



As an illustration of the harmless class of illusion, irradiation 



