Popular Science Monthly/Volume 71/September 1907/Mars as Seen in the Lowell Refractor
| MARS AS SEEN IN THE LOWELL REFRACTOR. |
By G. R. AGASSIZ
THE writer has lately enjoyed the great privilege, as Professor Lowell's guest, of observing Mars through nearly one presentation,[1] in the great 24-inch refractor.
Few people have had the opportunity of observing Mars at Flagstaff, and there is much scepticism afloat concerning the character of the markings of the planet, more especially as regards the double canals. So the writer proposes to give a short account of what can be seen, in the Lowell refractor, in one presentation, by any one of good eyesight, who is somewhat familar with the use of a telescope. The writer also wishes to give a description of the methods employed in observing, and the reasons for using them. He will also give a few reasons, which appear to him conclusive, to show that the double canals are actual phenomena, and not the result of diffractive effects in the telescope.
Few astronomers appear to realize how exceptionally excellent the seeing is in the clear dry air of Flagstaff, on a quiet night. It is so good, in fact, that a comparative novice appears to be able to see the planet more distinctly in one presentation there than Schiaparelli, at Milan, ever did.
During the time of the writer's observations, the diameter of Mars increased from 12" to 18". The eyepiece used in observing was usually a 25 mm. orthoscopic, Zeiss, which gives a remarkably large flat field. This gives, on the 24:-inch refractor, a power of 393. So that the apparent size of the disk of Mars was about 2.6 times the diameter of the Moon, as seen by the naked eye, at the beginning of the writer's observations, and 3.9 times at the end.[2] This is amply large enough to distinguish a vast amount of detail, when the seeing is sufficiently good to disclose it. Sometimes, when the seeing was unusually good, an eyepiece of 20 mm. would be tried, giving a power of about 490; but this was rarely used to advantage. When the seeing required a less power than the 25 mm., the planet could not be observed satisfactorily.
A circular disk was fitted over the eyepiece, containing an assortment of orange-yellow, and neutral-tinted glasses; any one of these could, at will, be revolved in front of the eyepiece. These glasses serve in a marked degree to bring out the contrasts on the planet. How little effect chromatic aberration plays in the observation of planetary detail may be judged from the fact that all the observers at Flagstaff preferred a neutral-tinted glass to a monochromatic one.
The action of a shade in bringing out detail appears to be somewhat as follows: In viewing a point of light through a telescope of a given aperture, the first minimum of the curve of diffraction, or the middle of the first dark ring, will always be at a given distance from the point of light, but the spurious disk will fade away, out of sight, before it reaches the minimum; and the fainter the point of light, the smaller the spurious disk. As the point of light approaches invisibility in the telescope, the spurious disk approaches zero. Now suppose we consider the light bordering a dark line on Mars as made up of numberless points of light. These points of light are excessively faint, compared with points of light on the sun, or with the light from a star. Their spurious disks are therefore extremely small, so that very little light spills over on to the dark markings; and that is the reason we are able to make out such fine detail on the planet. Now, although small, the spurious disks of these numberless points do diffuse some light on the fine dark markings. By using a shade, we decrease the light from these points, and thus reduce the size of their spurious disks. Therefore less light falls on the dark markings, and the sharpness of their edges is increased.
It is further found at Flagstaff that diaphragming the aperture increases the seeing. Langley, in his article on soaring birds, has shown that there are constant small changes of velocity "within the wind." Now these pulsations must cause waves of rarefaction and condensation, which may be represented as an irregular wave curve, sweeping past the objective. This will cause the planet to swing in the field of the telescope, as the rays are refracted by a layer of denser or rarer air. Now it is evident that the smaller the aperture of the objective, the less variation of the curve will there be in front of the objective at any given instant, or the more homogeneous the air in the path of the rays entering the eye at any given moment. So that, though a smaller objective will not diminish the swinging of the planet in the field, it will diminish the blurring within the planet, and help bring out the detail. Thus, the smaller the objective, the better the seeing, other things being equal. In practise the best results were obtained with a 12-inch diaphragm, as below this the loss of light and of effects due to increasing the size of the spurious disk began to be more powerful factors than the advantages gained from better seeing.
So importantly essential are the shaded glass, and the diminution of the aperture to the study of Martian detail at Flagstaff, that without these aids it would be excessively difficult to make out any of the fine detail on the planet.
It is a mistake to suppose that an observer who has a very keen sight for a small star will necessarily be a good observer of planetary detail. Indeed it often seems to be the reverse; as if an eye, sensitive to light, were not as acute for form. Now it is well known that no two objects can be separated by the human eye that do not fall on more than one cone in the fovea, or central pit of the retina. So may it not be that an eye, very sensitive to light, has unusually large cones in the fovea, while one acute for form has small ones?
There seems to be a great reluctance to accept the finer markings on Mars as established facts, and their objective reality has been questioned by all kinds of doubts and theories by all sorts of men. It might be well if some people, who explain away the markings on the supposition that they are optical illusions, would take the trouble to follow up their theories, see where they lead to, and work out what the appearance of the markings would be if due to the causes they suggest.
A Professor Douglass, of Arizona, has lately suggested that the canals are nothing but the black rays that can be seen radiating from a black spot, on a light ground, when looked at with a small screen placed in front of the pupil of the eye, so that the light enters only around the edges. According to Professor Douglass, the black oases are the only real things in the canal system, while the canals themselves have no tangible existence, and are nothing more than these black rays issuing from the dark spots.
In the first place, no eyepieces, constructed on any such principles as Professor Douglass uses to see these rays, are known at Flagstaff. Furthermore, the oases are more difficult objects to see than the canals. The latter are often visible when the former are not. It would seem that even Professor Douglass should find it hard to admit that, at such times, the canals are visible radiations from invisible spots.
But let us see what the canals would look like if actually due to this cause. These radiations are due to irregularities in the crystalline lens, and are constant for an adult, but vary with each individual. So that any one, looking at the planet, would see an exactly similar set of radiations issuing from each oasis. The planet would be covered with a quantity of black spots, all with similar radiations, and all absolutely independent of each other. For no two radiations from different spots would ever, except by the rarest chance, run into each other to form a straight unbroken line, connecting the two spots. The radiations would also be entirely different for each individual. As one of the most striking features of Martian detail is the manner in which the canals connect and interlace the oases, further comment seems unnecessary.
It has also been suggested that the so-called canals may not be lines at all, but merely a disconnected string of broken markings, a sort of irregular dotted line.
A series of observations at Flagstaff, conducted by several individuals, has shown that the eye is extremely sensitive to a break in a line. A series of lines,.3 mm. wide, was viewed from a distance of 17 feet. Each line was made up of 10 mm. sections, separated by small intervals that were the same between the sections of each line, but differed for every line. It was found that a line, whose sections were.5 mm. apart, was visible as a discontinuous line. A line with the sections .25 mm. apart appeared continuous.
With the power usually used at Flagstaff, the first figures would correspond on the planet at opposition to a line five and a half miles wide[3] visible as a discontinuous line, if the sections were eight miles apart. That the Martian markings should be composed of a series of dotted lines, separated by intervals never greater than eight miles, would seem far more wonderful than the canals themselves.
There is a wide-spread feeling that the double canals are due to diffractive effects in the telescope. The writer wishes to state, at soine length, why it appears to him that this can not be the case.
The writer has made many experiments, with various telescopes, on dark lines on a light field viewed by reflected light. In no ease has he been able to detect diffractive effects that in any way resemble the double canals of Mars, as seen in the Lowell refractor, while, on the other hand, parallel lines, close together, bear a striking similarity to the double canals.
In dealing with this subject, it is surprising to find how little is really known of diffractive effects caused by a dark line on a light field. This is the gist of the whole matter, and is a very different thing from the well-known effects of diffraction obtained when viewing a point, or a line, of light on a dark field.
In viewing a luminous point on a dark field through a given telescope the distance of the rings of diffraction from the center of the spurious disk may be easily found from the formula , where is the angle measured from the objective to the focus; c is a constant for each maximum or minimum; is a wave-length, and r is the radius of the objective. Now the second maximum, or the radius of the first bright ring, measures about 0″.31 in the Lowell refractor. If we extend this point to form a line, the ring will be transformed into two lines, one on each side of the source of light, at a distance of 0″.31 from it, and, since the second maximum has but.017 the intensity of the first, the outlying lines will be but.017 of the brilliancy of the central one.
On Mars we have to consider dark lines on a light field, and little seems to be known of their diffractive effects. There is a disposition to assume that we are here dealing with an inverted diffraction curve. Personally, it seems to the writer that there is no similarity between a bright line, whose light waves produce diffractive effects, and a dark line that emits no light waves. But let us assume that, somehow, the dark line on a light field will produce the same diffractive effects as a light line on a dark field. Then, were the double canals due to this diffraction, they would appear as follows on the planet, when seen in the Lowell refractor. Each and every canal would appear triple, the outer lines would always be separated by 0″.31 from their primary, and be.017 less distinguishable than it. Furthermore, there would be a dark ring around every oasis. No triple canal has ever been observed on Mars, nor has any ring ever been seen around an oasis.
The distance from the first minimum to the second maximum on the diffraction curve measures about ″.08 in the Lowell refractor. Now if the double canals were dark bands of a width of ″.16, then the points of light on the planet, at such a distance from the band that their first minima fell on its edge, would cast the light of their second maxima in the center of these bands, and these maxima, from the points on each side of any band, would overlap.
It is conceivable that such an effect might look something like a double canal, were it not for the fact that the diffracted light from all the other neighboring points of light would swamp and drown any such illusion. Supposing, however, that the double canals were really such dark bands, illuminated in their centers by the second maxima of the fringing light, then the double canals would always appear very nearly 0″.16 apart, which would correspond to about 1°.5 on the planet, when its diameter was 12″. But as the planet approached, since the distances apart of the maxima and minima in the focus of the telescope remain constant, the widths of the bands would no longer fit them, and the effect would be lost. Thus it follows that these bands of uniform width could never appear double, except at one given distance of the planet.
There are certain rules that the double canals should observe if they were due to diffraction, but they follow none of these. They should (since the size of the rings of diffraction remain constant through a given aperture) appear nearer together, in degrees on the planet, as Mars approaches; instead of which they remain the same size. They should (as diffractive effects vary in size inversely as the radius of the objective). as the objective is diaphragmed down, appear farther apart; but in fact diaphragming has no effect on their width. Not only should all the canals appear double, but they should all seem the same width. Less than one eighth of the canals have ever been seen to be double; and the double canals vary from each other in width, ranging from 2° to 5°.
In drawing A: 1 (the Euphrates) appears much wider than 2 (the Astaboras), while 3 (the Protonilus) is narrower than either; 4 (the
Vexillum), which is a double canal, the writer was unable to resolve, but he could never have classed it with 5 (the Astrusapes). This last appeared as a sharp dark pencil mark, as, indeed, do all the single canals, when the seeing is really good. The double canals then come out like the lines of a railway track seen from a half-distant hill.
If the double canals are really due to diffractive effects, how is it that only those are able to distinguish them whose eyesight is sufficiently good to obtain an exceptional view of the planet? Should not any one who can see the single canals be able to see them double? However, these remarks are probably quite useless. No one of good eyesight, who has seen Mars at Flagstaff, on a night when the seeing is really good, needs any arguments to convince him that what he sees is real. And no one who has made up his mind beforehand, without seeing them, that the double canals are due to diffraction, is likely to be influenced by these words.
It would seem almost unnecessary to state that no one for a moment supposes that the lines that one sees are actual streams of water. They are thought to be broad stretches of vegetation, dependent on channels of water running through them. So would the valley of the Nile appear to a distant observer, who would distinguish the dark fertile valley against the sands of the desert, long before he could see the river itself.
During the writers visit to Flagstaff, he saw 77 canals, 20 oases, and 11 double canals,[4] all of which, with one exception, could be readily identified on some one of Lowell's maps, though it was sometimes necessary to consult a map of an earlier date than the opposition cf 1905, to find them. Each of the drawings is the accumulated result of some 15 or 20 minutes at the telescope, so that no one of them represents ever}i;hing seen in a single night.
It must not be imagined that any drawing represents what the observer sees the moment he looks through the telescope. Instants of exceptional seeing flash out, here and there, at different spots on the planet. It is not till the same phenomena repeat themselves in the same way, in the same place, a great number of times, that the observer learns to trust these impressions. One has to keep one's mind constantly at the highest pitch to catch and retain what the eye sees.
It is like looking at a Swiss landscape from a high Alp, with the summer clouds sweeping about one. Now the mist rolls away, revealing a bit of the valley, and shuts in again in a moment: while in some other spot the clouds break away, and disclose a jagged summit, or a portion of a shining glacier.
Any one who has been fortunate enough to have had a really good view of the lineal markings on Mars is bound to be much impressed by their artificial appearance. So that, unless he has an inborn prejudice against the idea, any theory that accounts for the canals as the effort of intelligent beings to accomplish some definite object will not appear fanciful.
Lowell's theory that we have hero evidence that the inhabitants of Mars are struggling to preserve their existence by a planet-wide system of irrigation seems to be gaining ground; although he has had to contend against something of the same opposition that confronted Copernicus, Bruno and Galileo, and for very much the same reasons. The human mind resents anything that tends to belittle it, or its surroundings, and will not tolerate the idea of a rival.
It would seem, in all fairness, that a theory that fits all the observed facts as beautifully as Lowell's does deserves something better than disdainful disrespect, even from the most conservative. It is certainly far better than any theories and objections that do not meet the facts at all. As yet no other theory has been suggested that in an}' way accounts for the Martian markings. Until one is evolved that accounts for the facts better, Lowell's theory should Le accepted, by the most sceptical, as the only working hypothesis yet devised.
A very noteworthy achievement in the recent study of Mars is the series of remarkable photographs of the planet, taken by Mr. Lampland at Flagstaff, already he has succeeded in photographing many of the canals, and at the date of this writing[5] he has just photographed the Gihon double.
It seems as if, with the methods at present available, we probably shall not greatly increase our present knowledge of the planet. Even photography will probably be useful chiefly as a means of convincing the sceptical. But who can tell what the future may have in store? What astronomer of the early nineteenth century would have dreamed of the possibility of detecting, the composition of the stars, or determining their velocity in the line of sight? Some day a new method may increase our knowledge of Mars, as much as the discovery of the spectroscope opened up the heavens.
To most people "the proper study of mankind is man." But to those of us to whom the fact that we believe we have detected evidence of intelligent life in another planet seems of absorbing interest, Mars appears by far the most fascinating object in the heavens.
- ↑ From April 28 to June 2, 1907.
- ↑ With this power Mars appears about 5.2 times the diameter of the moon, at opposition in 1907.
- ↑ Various observers have experimented at Flagstaff at different times with wires stretched against the sky, viewed at ever increasing distances. The distances at which a wire was distinctly visible varied with different individuals, and corresponded to an angular width for the wire varying from.69″ to.93″. Looking at Mars at opposition, with a power of 400, these angles would correspond on Mars to widths of from 0.31 to 0.42 miles. Making all possible allowance for loss of light, etc., in the telescope, it seems probable that, under favorable circumstances, a line less than a mile wide could be detected on the planet. By comparison with the finest micrometer threads, some of the single canals are estimated to be as much as 35 miles wide. The width of the various double canals, which remains constant for each canal, is estimated to range from 2° to 5° on the planet. This is found, by various terrestrial experiments at Flagstaff, to be far wider than lines that can be easily separated by the eye.
- ↑ There are known at present 436 canals, of which 51 are double, and 186 oases. These are never all seen at one opposition, not only because of the different tilts of the planet, but also because neighboring canals alternate in appearing and disappearing at different oppositions. Accepting Lowell's theory that the canals are areas of vegetation bordering artificial channels for irrigation, this could be accounted for by the fact that when the canals do not appear, the land is lying fallow.
- ↑ July, 1907.