128 SCIENCE PROGRESS 



order to be assured of its reality. He will thus find out what 

 to seek and believe. It is an old story. To be discovered, a fact 

 must force an entrance into the stronghold of men's minds ; 

 when once it has achieved this it becomes a welcome guest. 



This fact has been already exemplified in the case of Mars. 

 His satellites required a twenty-six inch telescope and persistent 

 care for their discovery but have often since been seen with 

 telescopes of less than half this size. 



Although the more salient details of the disc of Mars may 

 be corroborated by any observer who has the needful practice 

 and patience, the discrimination and discovery of the more 

 intricate and minute parts require special qualifications which 

 few possess and practice cannot give them. I refer to the 

 intrinsic defining power of the observer's eye considered as an 

 instrument. 



Lowell has pointed out that there are two useful extremes 

 in eyesight which cannot meet — defining power and sensitive- 

 ness to light. Suitable education of the eye assists by drawing 

 the two extremes nearer together but the possession of either 

 quality in a superlative degree excludes the other. 



In the retina on which the image falls there is a structure 

 of rods and cones varying markedly in size and texture in 

 different eyes. Those having the finer texture have also the 

 greater defining power but are deficient in sensitiveness. A 

 photographic analogy may help. Rapid plates are more sen- 

 sitive to light and of coarser grain than the slower plates 

 which give a sharper picture. The increased definition on the 

 slower plate is due to the fact that the finer grain produces 

 less distortion of the detail which falls upon it. 



To return to Mars. We find at once among observers of 

 the planet a striking contrast. Prof. Barnard, who by his 

 discovery of the fifth satellite of Jupiter (an object of excessive 

 faintness) proved the sensitiveness of his eye, finds himself 

 entirely unable to detect any of the " canali " which are so 

 evident to Lowell. 



Of course some ol this discrepancy is due doubtless to 

 differences of climate and instrument but there remains a 

 residuum which can only be explained by a difference of eye- 

 sight. Fortunately for the elucidation of the problem many — 

 like the writer — possess eyes intermediate between these two 

 extremes, so that to some extent they may share the discoveries 



