CONDITIONS OF HABITABILITY OF A PLANET. 



91 



so artificial ? And why is there this discordance between his 

 observations and those of other astronomers at least as skilful 

 and experienced, and with equipment certainly not inferior ? 



The " Ancient Mariner," in Coleridge's poem, describing the 

 approach of the phantom ship to the " Wedding Guest," says : 



" At first it seemed a Uttle speck, 

 And then it seemed a mist, 

 It neared and neared, and took at last 



A certain shape, I wist. 

 A speck, a mist, a shape." 



There could scarcely be a neater way of stating the solution 

 of the problem. When the phantom ship was first detected on 

 the horizon it was too far off to give any idea of form or 

 outline. It was unmistakable that something was there, but 

 the Ancient Mariner could see nothing but a " speck," a round 

 dot ; it was too far off to show any detail ; the details were all 

 averaged out, and it formed a minute circular spot. 



And then it neared, and it was clear that it had details, but 

 what they were the Mariner could not say ; it was an ill- 

 defined, shapeless object, " a mist." And again it neared, and 

 then it began to take a " certain shape " ; he could recognize the 

 hull, the mast, the spars. 



In 1830, the two German astronomers, Beer and Madler, 

 observing Mars with a telescope of 4 inches aperture, 

 frequently drew two round spots on the planet, exactly the 

 same size and exactly the same shape. Thirty-four years after- 

 wards those spots were drawn by Sir Norman Lockyer with a 

 telescope of 8 inches aperture, but neither of them was round, 

 and they bore no resemblance to each other. A few years 

 later Schiaparelli drew them with a telescope of 18 inches 

 aperture, and both spots were then full of minute detail, and 

 more unlike each other than ever. In 1909, M. Antoniadi 

 observed both regions with a telescope of 33 inches aperture 

 and added yet more detail and further increased their unlike- 

 ness. Now these changes in the representation of the planet 

 are not due to any change on the planet itself. An observer 

 coming fresh to its study and having a telescope of only 

 4 inches aperture, will see exactly what Beer and Madler did 

 under the same conditions — two round dots exactly alike. But 

 if he carefully train himself, and increase the size of his 

 telescope, then, granted he possesses the eyesight and skill of 

 the astronomers I have mentioned, he will give us in succession 

 views that practically cori'es{)ond with those of Lockyer, 

 Schiaparelli and Antoniadi. Tlie increase in telescopic power 



