Some interesting observations were made by Professors Payne and H. C. Wilson, in the summer of 1904, at Midvale, Montana (U.S.A.), at a height of 4790 feet above sea-level. At this height they found the air very clear and transparent. "Many more stars were visible at a glance, and the familiar stars appeared more brilliant.... In the great bright cloud of the Milky Way, between β and γ Cygni, one could count easily sixteen or seventeen stars, besides the bright ones η and χ,[277] while at Northfield it is difficult to distinctly see eight or nine with the naked eye." Some nebulæ and star fields were photographed with good results by the aid of a 2½-inch Darlot lens and 3 hours' exposure.[278] Prof. Barnard has taken some good stellar photographs with a lens of only 1½ inches in diameter, and 4 or 5 inches focus belonging to an ordinary "magic lantern"! He says that these "photographs with the small lens show us in the most striking manner how the most valuable and important information may be obtained with the simplest means."[279] With reference to the rising and setting of the stars due to the earth's rotation on its axis, the late Sir George B. Airy, Astronomer Royal of England, once said to a schoolmaster, "I should like to know how far your pupils go into the first practical points for which reading is scarcely necessary. Do they know that the stars rise and set? Very few people in England know it. I once had a correspondence with a literary man of the highest rank on a point of Greek astronomy, and found that he did not know it!"[280] Admiral Smyth says, "I have been struck with the beautiful blue tint of the smallest stars visible in my telescope. This, however, may be attributed to some optical peculiarity." This bluish colour of small stars agrees with the conclusion arrived at by Prof. Pickering in recent years, that the majority of faint stars in the Milky Way have spectra of the Sirian type and, like that brilliant star, are of a bluish white colour. Sir William Herschel saw many stars of a redder tinge than other observers have noticed. Admiral Smyth says, "This may be owing to the effect of his metallic mirror or to some peculiarity of vision, or perhaps both."[281] The ancient astronomers do not mention any coloured stars except white and red. Among the latter they only speak of Arcturus, Aldebaran, Pollux, Antares, and Betelgeuse as of a striking red colour. To these Al-Sufi adds Alphard (α Hydræ). Sir William Herschel remarked that no decidedly green or blue star "has ever been noticed unassociated with a companion brighter than itself." An exception to Herschel's rule seems to be found in the case of the star β Libræ, which Admiral Smyth called "pale emerald." Mr. George Knott observed it on May 19, 1852, as "beautiful pale green" (3·7 inches achromatic, power 80), and on May 9, 1872, as "fine pale green" (5·5 inches achromatic, power 65).