1910] on Lotuell Odservatory Photographs of the Planets. 817 



no detriment. Rather the reverse, for it furnishes a greater scale for 

 measurement. 



In looking at the photographs two things must be borne in mind. 

 One is that the irregularities due to the grain of the plate must not 

 be attributed to the images. Thus, within the limits set by the 

 grain the lines on Mars show as Knes, not as a patchwork. This is 

 perfectly apparent when they are carefully scanned. When we 

 consider that the original images are only 5 mm. in diameter we 

 reahse the strain of lantern exhibition. Even so they are magnified 

 200 times in the taking. They are then further enlarged on the 

 shde, and lastly thrown greatly increased upon the screen. The 

 wonder is that they stand this limelight publicity at all. 



The second point is that we are not dependent on them for our 

 minute knowledge of the planet. A good eye trained to the subject 

 sees at least ten times as delicately as the film. But it must be an 

 eye suited to planetary work, which is quite a different eye from that 

 good at faint satellite or nebula detection. It is very important to 

 remember this, for not only is there a physiologic reason for it, but 

 mistake of it is often made in high quarters. When an observer 

 records a polar flattening as twice and four times what hydro- 

 dynamics permit, his forte lies elsewhere than in planetary research. 



Three planets will now show you their presentments : Mars, 

 Jupiter and Saturn. I was minded at first to omit Mars, passing by 

 this old acquaintance with a nod, but so great have I found the 

 interest in him here as elsewhere that he has been put beside the 

 others. 



As an example of the delicacy of the detail to be descried on 

 him, not only by the eye but in the photographs, may be instanced 

 the sight of one of the many vicissitudes of his changeful year, which 

 suddenly appeared one day when least expected. The event was the 

 first frost of the season in the Antarctic regions of Mars, detected 

 visually at Flagstaff on November 16. The patch was at once photo- 

 graphed, and is plainly apparent on the plate. To chronicle thus 

 the very weather on our neighbour will convince anyone that inter- 

 planetary communication has already begun, and that, too, after the 

 usual conventional manner of ordinary mundane greetings. 



My next mention shall show you the pitch of precision to which 

 measurements of these little prints can attain. It is well known 

 that the south polar cap of Mars is not centred on the pole, but lies 

 some 6° off it, in longitude 20° or thereabouts. When the images 

 showing the cap at two different longitudes were measured the 

 measures revealed distinctly the excentring of the cap, and even 

 registered with some accuracy its amount and position. When we 

 reflect what this means, it looks as if Mr. Crommehn's belief that 

 areology would stand indebted to the photographs for help in its 

 geodetic survey is in a fair way to be realised. 



It would be possible in these photographs to take you on many a 



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