February 27, 1908] 



NA TURE 



403 



blue rays would have to be made use of, and they would 

 irretrievably blur the image. Plates sensitised to other 

 parts of the spectrum must be employed, and as it has 

 not been possible to make such adapted only to the yellow 

 and orange rays, a coloured screen must be used in con- 

 nection with them. Only when more restricted emulsions 

 shall have been produced will it become feasible to dis- 

 pense with the sifter. 



The plates most nearly giving us what we wanted proved 

 to be Cramer's isochromatic instantaneous plates. They 

 are neither instantaneous nor isochromatic, but their two 

 negatives nevertheless combine to the best affirmative it 

 is now possible to obtain ; for beyond their mountain 

 mass of reaction in the violet and blue, to speak figura- 

 tively of their curve of sensitiveness, they have a hillock 

 in the yellow w'ith sides of great abruptness. By Mr. 

 Lampland, who carefully e.xperimented with every kind 

 of plate, these were found, and for the above reason, to 

 yield the best results in the way of speed -with definition. 

 Their chief drawback consists in their not being so finely 

 grained as one could wish. 



This brings us to another difficulty that had to be 

 encountered. In the sensitising of plates speed is inevit- 

 ably associated with coarseness of grain, and "oicc -jcrsA. 

 Finely grained emulsions are necessarily slow. To avoid 

 the Scylla of over-exposure is to fall into the Charybdis 

 of undcr-definition. As speed must be got at all hazards, 

 the images of Mars are not so fine in texture as those 

 securable of earthly scenes, as, for example, by lantern- 

 slide plates. This is to be remembered in scanning the 

 images. Anything beyond a slight magnification of the 

 original negatives , results in perceptible, though not 

 always perceived, blurring of the details due to the show- 

 ing of the grain. Photographic experts will thoroughly 

 appreciate this, and trace the linearity of the canals 

 clearly through its partial disguise. 



.\fter all other points have been attended to, there still 

 remains the question of aperture ; for the smaller the 

 aperture the sharper the definition for the same sized 

 magnification, the gain from the point of view of the air- 

 waves vastly exceeding the loss due to a larger spurious 

 disc. Thus with an image magnified to four hundred 

 diameters, a 12-inch gives ninety-nine times out of a 

 hundred more clear-cut detail than a 24-inch. This, which 

 is so decided a gain in visual work, is partially offset in 

 photographic work by the necessary increase in exposure 

 time, and the consequent greater chance of mixing poor 

 moments with the good. As the exposure time decreases 

 inversely as the square of the radius of the glass, while 

 the improved definition increases inversely as that radius, 

 Mr. Lampland, in 1907, used only the full aperture of the 

 24-inch. In this decision I quite concur from the results 

 at the last opposition, merely adding the gloss that with 

 the larger aperture one is more certain of a good image : 

 with the smaller, one will score an even greater success 

 on exceptional occasions. This, of course, is to be taken 

 within limits. 



By the general method I have outlined Mr. Lampland 

 secured the first photographs of the canals at the opposi- 

 tion of Mars in 1905. Some fifty of the canals show upon 

 his plates. This success was entirely due to his exhaustive 

 studv and attention to all the factors I have formulated. 

 During the time between 1905 and 1907 Mr. Lampland 

 continued his research, and in June, 1907, took his first 

 plates at the opposition just passed. The images showed 

 a marked advance. In 1905 he had registered the 

 Nilokeras double (r2° apart). In June, 1907, the Gihon 

 f5°.o apart) stood duplicate in his photographs, while the 

 Euphrates (4° apart) pretty certainly showed in the same 

 manner, though from principles of conservatism I was 

 not willing to commit myself to its announcement. At 

 the same time plates were taken by me showing in like 

 manner a great number of single canals, and the double 

 Gihon and almost unquestionably the double Euphrates 

 appear. For the same opposition an expedition was under- 

 taken bv the Lowell Observatorv to the Andes under the 

 charge of Prof. Todd, Mr. E. C. Slipher, of the Lowell 

 Observatory, being detailed upon it armed with a duplicate 

 of our apparatus, and thoroughlv coached beforehand by 

 Mr. Lampland in its employ. Mr. Slipher, bv whom all 

 the work of the expedition on Mars, both photographic :ind 



NO. 2000 VOL. 77] 



delineatory, was done, secured plates in July at Alianza, 

 Chile. The place had been chosen primarily because of 

 its lying in the solar eclipse belt, Prof. Todd being 

 desirous of observing the annular eclipse there, but it 

 proved, although but 4000 feet above sea-level, probably 

 the best locality that could have been selected. By a skill 

 and assiduity deserving of the highest praise, Mr. Slipher 

 obtained some 10,000 images of the planet in the course 

 of a couple of weeks. Owing to the remarkable steadi- 

 ness of the air and the high altitude of the planet, his 

 plates show a wonderful amount of detail. The doubling 

 of the Gihon and of the Euphrates previously registered 

 at Flagstaff were also exhibited on them, besides canals 

 and oases in profusion. As an instance of the latter I 

 may mention the distinct showing of the two little oases 

 in 'the Trivium. His drawings were no less remarkable. 

 .'\s an example, the double Ganges, which for two opposi- 

 tions now the writer has observed stronger on its right 

 or western side than on its eastern, appears with this 

 same differentiation in Mr. Slipher 's picturing, although 

 he had no previous acquaintance of the fact. Havmg 

 discovered that he has an eye for planetary detail, he is 

 to continue such study in the future. 



The future promises even more than the past has 

 fulfilled. Several improvements have been effected, or are 

 in contemplation, which were not put into operation at 

 the opposition of 1907. One of them is a new screen 

 devised by the writer. Though both conceived and con- 

 structed before the opposition, it was only tested this last 

 summer, but enough to show an improvement in definition 

 from its use. Its basic principle was the integration of 

 the greatest amount of illumination with the least focal 

 difference of wave-length. To explain the idea, suppose 

 that the light reaching the plate for each ray be weighted 

 according to its proximity for focal length to a given focal 

 distance by an inverse function of such departure, the 

 function becoming negative after a certain discrepancy 

 because the inclusion of the ray then does more harm 

 than good. Suppose this light summed for all the rays 

 between certain limits. The most effective screen will be 

 that for which the integral is a maximum. The point up 

 to which the rays should be cut off, as indicated by an 

 examination of the colour curve of the 24-inch glass, 

 seemed to lie at ^ = 5000, and for this, accordmgly, I 

 asked Mr. Wallace to construct a special screen. The 

 result, though for some reason not so effective practically 

 with bathed plates as was theoretically to be expected, 

 proved successful with Cramer's isochromatic instantaneous 

 plates owing to the insensitiveness of the plate for the red 

 and uitra-red rays, and to the fact that ^ = 5000 marked a 

 minimum in its action followed by a rise.' 



Other devices which should improve the process are also 

 to be practised, and these, with the Increased presentation 

 of the planet's disc, should result in another decided 

 advance in photographic presentment; for the planet w^iU 

 in 1909 be more advantageously placed for Flagstaff on 

 three counts : — 



fi) .\ larger disc. 



(2) .\ greater altitude. 



(3) .'V more developed condition of the canals due to the 

 advance -in the Martian season. 



In spite of the interest which the taking of such photo- 

 graphs has caused, it must be remembered that after all 

 the eve remains our most potent instrument of research. 

 So thoroughlv was this realised at Flagstaff th.at the 

 photographs were originallv undertaken simply with a 

 view to their educational value. Inasmuch as these photo- 

 graphs in good air are superior to untrained eye observa- 

 tions in a poor one, thev serve to dispel directly a modicum 

 of doubt, though thev 'cannot at present equal what the 

 trained eve can see ' under similar conditions. But m- 

 directlv they do more ; for they corroborate completely, so 

 far as they go, visual observations which have been so 

 extensively 'de"nied, and establish, therefore, a very strong 

 presumption that those visual detections are true also 

 bevond what the photographic plate has power to portray. 

 In' this connection it is interesting to note that more 

 than one astronomer who has seen the canals from a 

 middle ground of definition neither good nor bad has 

 1 For a more detailed account of the device, see the Lowell Observatory 

 Bulletin, No. 31. 



