162 



THE NATIONAL GEOGRAPHIC MAGAZINE 



in its apparent nightly path at that mo- 

 ment. He pulls a switch, and the big 

 instrument sweeps along the star's well- 

 beaten track until its approximate posi- 

 tion is reached. The slow motion is 

 brought into play, and the big barrel 

 swings directly on the star, which the 

 clockwork, in turn, causes the telescope 

 to follow as it journeys across the 

 heavens. 



Suppose that with your merry-go- 

 round spyglass you should have two 

 spider threads crossing one another 

 at right angles, and that the house you 

 were looking at was a mile away ; and 

 then suppose that the glass was so pow- 

 erful that you could see the head of a 

 nail at that distance ; and then further 

 suppose that you kept the intersection 

 of the two spider threads trained on 

 that nail-head. Then you have a fair 

 measure of the delicacy of the adjust- 

 ments of the Xaval Observatory, Yerkes, 

 and Mount Wilson telescopes. 



Formerly the floor of the observatory 

 was stationary, on a level so low that 

 when the instrument was pointed at the 

 zenith a man sitting in an ordinary chair 

 could look into the eyepiece : but when 

 looking at a star nearer the horizon the 

 observer had to climb up a glorified step- 

 ladder twenty or thirty feet high and 

 observe his star from such an unstable 

 perch. 



Now,' however, the floors of modern 

 observatories can be raised and lowered 

 like an elevator. The domes are made 

 to revolve, so as to bring the shutter- 

 opening over the object end of the tele- 

 scope (see page i 6t ). 



TAKING PICTURES OI' DISTANT WORLDS 



Many of the star observations are not 

 made with the eye. A majority of them 

 arc- made with a photographic attach- 

 ment. Often a photographic plate on 

 the big telescope will record in minutes 

 what would require days to work out 

 with eye observations. At the Mount 

 Wilson Observatory some photographs 

 are taken that have to be exposed for 

 four nights. 



Think of the wonderful perfection of 

 a driving clock that makes possible four 

 all-night exposures of a given group of 



stars, no adjustment being required for 

 speed, but the photographer having to 

 keep a constant watch for such changes 

 as the quality of the air, so as to adjust 

 the instrument to meet them ! 



Powerful as the big telescopes are, 

 they have their limitations. An instru- 

 ment that magnifies six thousand diame- 

 ters might be employed, theoretically, in 

 low-altitude work. Such a telescope 

 would bring the moon to a distance of 

 only forty miles. 



ATMOSPHERE} LIMITS THE TlX£SCOP£ 



But the power that would bring the 

 moon so close, except on high mountains, 

 would also magnify greatly the tendency 

 of the air to obstruct our sight ; and, as 

 the late Dr. Simon Newcomb once said, 

 the moon might be brought that close, 

 but our view of it would be as though 

 we were looking at it through a tiny 

 pinhole and several yards of running 

 water. Under such a view the whole at- 

 mosphere would look like the air over a 

 hot automobile engine or above a stove — 

 full of heat waves. It is those waves 

 that cause the fixed stars to twinkle. 



The observatories on mountains and 

 high plains get rid of so many atmos- 

 pheric difficulties that it is possible to 

 magnify one hundred diameters for each 

 inch of diameter of the mirrors. The 

 big ibo-inch reflector on Mount Wilson 

 therefore has a magnifying power of ten 

 thousand diameters. In other words, an 

 object two miles distant would appear as 

 big as if it were only 12^2 inches in front 

 of the unaided eye. The big mirror will 

 gather in a quarter of a million times as 

 many rays as the pupil of the eye receives 

 unaided. 



But next to the big equatorial tele- 

 scope in an observatory the spectroscope 

 claims chief interest. A wonderfully 

 versatile instrument it is in applying the 

 third degree to light. Light is composed 

 of waves of an infinite variety of lengths. 

 The shortest wave-length the eye can see 

 is 1/70000 of an inch long and the long- 

 est is 1/40000; yet the Annapolis Wire- 

 less Station makes use of wireless waves 

 more than ten miles long, and the Bureau 

 of Standards employs X-rays a billionth 

 of an inch short (see also page T58). 



