Page:Popular Science Monthly Volume 71.djvu/281

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HE writer has lately enjoyed the great privilege, as Professor Lowell's guest, of observing Mars through nearly one presentation, 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 familarfamiliar [sic] 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. 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.