4o: 



NA rURE 



[February 27, 1908 



PLANETARY PHOTOGRAPHY. 



^PHIi recording on photographic plates of the canals of 

 -'■ .Mars is as significant from a technical point of view 

 as it has proved of widespread interest in its result; for 

 the method which alone rendered success possible had 

 first to be developed, previous celestial photographic pro- 

 cesses being inadequate to the tasl-:. At the request of 

 the editor of Nature, I propose to give some account of 

 the method pursued, and the more gladly in that it is 

 evident from attempts to follow it that its principles are 

 as yet as much a terra incognita as have for so long re- 

 mained the canals themselves. The process is the outcome 

 of four years' study by Mr. Lampland, who, to a know- 

 ledge of the end desired, acquired from visual work on 

 the planet, added experimental research on the means to 

 attain it. Of the difficulty of the subject the best testi- 

 mony are the words Scliiaparelli wrote the writer on 

 receiving in 1905 the first prints from the plates : — " I 

 would never have believed the thing could be done." 



The fundamental distinction between planetary photo- 

 graphy and photography of the stars is 

 that with the former definition, not 

 illumination, is the primary point. To 

 imprint upon the plate such delicate tracery 

 as the canals of Mars requires a definition 

 so far beyond celestial photography in 

 general as to constitute a class of work by 

 itself. For one is here concerned with 

 quantities of the second order of minute- 

 ness. 



Definition, therefore, had to be studied. 

 The chief disturber of the image is the 

 atmosphere. A knowledge of how this 

 conditions the seeing is, then, the first 

 requisite to success. Living as we do 

 under a gaseous ocean in constant turmoil, 

 no image from beyond it stays perfect for 

 long, soon being either distorted or dis- 

 placed by the shifting refraction of differ- 

 ently dense layers of air. The effect we 

 notice every day in the twinkling of the 

 stars. To educate the eye to sift the fleet- 

 ing impressions it receives is thus the first 

 step to becoming an observer of Mars, 

 Distrust of its own revelations because of 

 their short-Iivedness is one chief cause of 

 failure to see the canals More, not less, 

 is a like handicap true of the camera ; for 

 the eye is some thousands ' of times as 

 sensitive as the films we can emplov. So 

 that at first it would seem hopeless to 

 attempt to part the good moments of 

 definition from the bad, and thus to pre- 

 vent the superposition of a poor or shifted 

 image upon a clear-cut one, to the result- 

 ing disheartenment of a general blur. 



To catch the planet's fugitive expressions 

 of itself, speed of exposure becomes 

 imperative ; and that as many such as 

 possible might be seized, a special camera had to be 

 devised, something which should realise the demon-machine 

 of Clerk-Maxwell for images in place of molecules, to let 

 the good ones through and stop the bad. The mechanical 

 part of this Mr. Lampland contrived by a plate-holder 

 fitted with a lateral ratchet motion worked by a bulb, and 

 capable of being pushed up and down after each line of 

 images had been secured. At the opposition of 1905 this 

 camera was worked without guiding, as the exposure 

 time seemed not to necessitate it, but for that of 1907 Mr. 

 Lampland suggested the use of, and fitted on, the 6-inch 

 as a finder. In spite of the very short exposure possible, 

 the guiding thus introduced turned out an improvement. 



The next difficulty in definition lies with the glass. 

 In spite of its name, no achromatic lens is achromatic. 

 Though the departure from perfection is practically 



imperceptible to the eye, such is not the case with the 

 sensitised film ; for the rays of different colours form 

 their images in different focal planes. Of these, the eye 

 selects what it will attend to, while the camera cannot, 

 and so, on the plate, if an image made by one colour be 

 in focus, it must perforce be surrounded by others that 

 are not. A reflector, of course, avoids this blur of super- 

 position, since all the rays are brought to one focus, but, 

 on the other hand, it introduces more serious errors of 

 spherical aberration ; for not only does any want of 

 figuring or of sag; in the mirror, but any disturbance in 

 the air produces three times the distortion it does in the 

 glass. It is thus problematic whether a reflector can ever 

 be used for such fine work, though we intend to give it 

 a trial with a 3-feet mirror. 



To secure approximate monochromatism, and thus a 

 more clear-cut image, a screen or filter of coloured glass, 

 or of a coloured solution between glass, had to be used 

 to cut off certain of the rays. This device is the same 

 that was used visually by Schiaparelli, and that has been 

 used at Flagstaff in like research, though it has not been 



phs of Mars ; Cinges region. Taken by Prof. Percival Lowell at Fbgbtaff, July 26, 1907. 



> With the Flagstaff objective diaphrasmed down to 

 a power of 3Q3 further weakened by a screen that take- 

 quarters of the light the eye sees on Mars' canal 

 twentieth of a second which it takes the plate two seconds to register 

 magnihcation of rso and under the full aperture of the 24.inch clas=. 



NO. 2000, VOL. 77] 



found there so effective as a neutral-tinted glass, because, 

 as mentioned above, the eye does its own sifting for the 

 rays it elects to observe. Photographically it was first 

 employed by Ritchey in his photographs of the moon, and 

 here its value is inestimable. The general method of 

 making the screens is to determine first the colour-curve 

 of the objective, that is, the curve in which the abscissa 

 represent the wave-lengths of the rays of dilTering refrangi- 

 bility and the ordinates their focal lengths. From this 

 curve it becomes possible to select what rays shall be 

 allowed to pass to secure a sufficient approximation to 

 monoch'romatisation, and the screen is then coloured to 

 attain the result. In the construction of such screens Mr. 

 R. J. Wallace is preeminent, and by him in this manner 

 were inade those for the Flagstaff glass. 



The next crux entered with the plates. In consequence 



of the greater relative deviation in focat length suffered 



- ,and with by the blue rays, which are the ones most actinic, and 



least three- those to which the ordinary plates are sensitised, such 



plates cannot be used for interplanetary photography. To 



get enough light with them to approach instantancity the 



