November, 1913. 



KNOWLEDGE. 



439 



had a whole-plate Sanderson camera practically ready for 

 action, and set this up on the stoep of my house (facing 

 northwards) as quickly as possible. As I had a focal-plane 

 back on the camera, I opened the slit to the width of the 

 plate and decided to expose by withdrawing the slide of the 

 plate-holder, the lens cap not being at hand. Generally in 

 photographing lightning one has to wait for a little time to 

 secure a good flash, but on this occasion there were a number 

 of bright flashes whilst I was in the act of withdrawing the 

 slide. The bright flash to the right of the centre of 

 Figure 507 was the first to occur. At the moment I was not 

 certain whether the flash was too early for me. It was 

 followed almost immediately by the great flash which appears 

 on the left of the picture, and on seeing this I closed the 

 plate-holder at once. The plate was exposed at 7.45 p.m., 

 and was developed at 8.5 p.m. on the same evening. The 

 centre flash is seen to be multiple, showing six or seven 

 discharges. The displacement of the individual flashes is due 

 to the fact that the flash occurred whilst I was withdrawing 

 the plate and the camera was oscillating slightly. The large 

 flash on the left of the picture also shows a similar effect. 

 This flash was much nearer to the camera than the two 

 flashes near the middle of the picture ; the latter are beyond 

 a low cloud, whereas the former is in front of this cloud. 



The successive strips into which the negative is divided 

 puzzled me whilst the plate was developing, but the explana- 

 tion soon suggested itself. They represent flashes which 

 occurred during the short interval occupied in opening (and 

 possibly also closing) the plate-holder, which were not in the 

 field of view of the camera, but were near enough to produce 

 a fogging effect. The time taken for the double operation of 

 opening and closing the dark slide I found was nearly three 

 seconds, and in this interval there were at least twelve flashes. 



Another plate was exposed at 7.50 p.m. on the same even- 

 ing, and as the storm was then much more distant the camera 

 was left open for some time. On developing, three small 

 flashes were seen to have impressed themselves (see Figure 

 506) and, very strangely, they were all photographically 

 different, i.e., one gave a normal image, the second a reversed 

 image, and the third a re-reversed or solarised image. 



H. E. WOOD (F.R.Met.S.). 

 Transvaal Observatory. Johannesburg. 



MARS. 

 To the Editors of " Knowledge." 



Sirs, — In your issue of August, there appeared a letter from 

 M. Antoniadi with reference to a letter of mine which appeared 

 in your June issue. In discussing the questions arising from 

 these letters, I will endeavour as far as possible to dispense 

 with arguments ad hominem, and confine myself to arguments 

 ad rem. 



M. Antoniadi declares that I misunderstood the purpose of 

 his article in the May issue of your journal (" Considerations on 

 the Physical Appearance of the Planet Mars," pages 193-196). 

 If my interpretation of this article (which is set out in my 

 letter in your issue of June) is incorrect, I had ample excuse for 

 being mistaken. The scorn which in this article M. Antoniadi 

 pours on those who are not of his way of thinking would, I 

 think, justify anyone in supposing that he claimed that his 

 views held the field in science. The motive which led me to 

 criticise M. Antoniadi's article was not an anxiety to contra- 

 dict, but a desire to point out to the uninitiated reader that his 

 views are by no means universally accepted by men of science. 



I am satisfied on the not very important point that there 

 are instances where the canals have been represented out of 

 perspective. Taking into consideration the extreme delicacy 

 of these objects, and indeed they are so delicate as to require 

 the most favourable conditions for observation, and even in 

 the fine air at Flagstaff a disturbed atmosphere at times blurs 

 them out of recognition, it is not surprising that errors of 

 this nature occasionally occur. 



M. Antoniadi seems to have altogether failed to understand 



my meaning when I showed (I hope satisfactorily) that it is 

 not legitimate under any circumstances to represent as 

 indistinct and vague an object which is seen as clear and 

 defined. It is quite possible that if the image of Mars in the 

 telescope could be so enlarged that with perfect definition it 

 appeared the same size as a drawing three inches in diameter 

 held at a distance of one foot, features which under ordinary 

 circumstances appear sharp might appear exceedingly vague 

 and undefined. But, on the other hand, it is also possible 

 that they would appear as sharp and defined as they actually 

 do under ordinary circumstances, with the possible addition 

 of smaller canalian features. This is a question which 

 cannot at present be decided. M. Antoniadi's analogy 

 of a sixpence is a false one. It is true that if a six- 

 pence (or almost any other artificial production) is greatly 

 enlarged its details become vague. This, however, is not 

 true of most objects of nature, in many cases the original 

 details remaining sharp and well defined, and finer features 

 coming into view. If a sixpence were struck of the size of 

 three inches, all the details being perfectly defined, and then 

 reduced to the size of an ordinary sixpence, an enlargement 

 of it to its original size would not show the details vague. It 

 is thus impossible to ascertain without experiment what the 

 appearance of an object will be on enlargement. In recording 

 observations by drawings it is therefore only legitimate to 

 draw exactly what one sees, and it is not legitimate to make a 

 drawing showing what one thinks the planet's appearance 

 would be if it could be seen as large as the drawing. Such a 

 method is drawing beyond one's vision. 



Then with regard to photography. In the perception of 

 light and shade contrast the camera is superior to the eye. 

 Hence the great value of the camera in the perception of faint 

 stars and strands of nebulosity and of the light and dark areas 

 of the planet Mars. But it must be admitted that for the 

 perception of fine, sharp detail the camera is not as efficient 

 as the eye. Far more detail has been seen on the moon than 

 has ever been photographed. Poor would be the observer 

 who could see on the moon no more detail than appears on 

 the very finest lunar photographs. In the Martian photographs 

 all the finer detail of the planet's marking is blurred almost 

 out of recognition. It is claimed, however, that canals are 

 indicated even on photographs. It is noticeable that the 

 photograph of Mars given on page 194 in your May issue 

 shows almost as much detail as does M. Antoniadi's drawing 

 of the same region. For light and shade studies of the planet 

 M. Antoniadi's drawings are, undoubtedly, worthy of the 

 highest admiration. 



It is useless to go again into a discussion on the advantages 

 of using large and small apertures. It is a matter which must 

 depend on practical experiment. Professor Lowell, after 

 many years' experience with a telescope of the finest make, 

 situated as M. Antoniadi admits in the finest atmospheric 

 conditions, appears to have found that his best results are not 

 obtained when he is using his full aperture of twenty-four 

 inches. 



It may be interesting if I make a quotation which refers to 

 Professor Lowell's observations from a book entitled " Is Mars 

 habitable ?" by Professor Wallace, F.R.S., who is as much 

 opposed to the theory of the artificiality of the canals as is M. 

 Antoniadi. He writes on page 14 of this book, in reference to 

 one of Professor Lowell's works, " No one can read this book 

 without admiration for the extreme perseverance in long- 

 continued and successful observation the results of which are 

 here recorded, and I myself accept unreservedly the whole 

 series." 



I believe that much of the dispute as to the existence or 

 non-existence of the canals as straight lines arises from a 

 confusion between acuteness of vision (or the perception and 

 separation of fine detail), and sensitiveness to impression (or 

 the perception of faint contrasts). The two qualities rarely 

 go together in a high degree though one is often erroneously 

 taken as a guarantee of the existence of the other. 



J. E. MAXWELL. 

 84, Dartmouth Road, N.W, 



