Then there are the stellar parallaxes, now observed for many stars at once photographically, when formerly only one star's parallax could be measured at a time and with the eye at the telescope. And photo-electric photometry, measuring smaller differences of light than any other method, and providing more accurate light-curves of the variable stars. And perhaps most remarkable of all, the radial velocity work on both stars and nebulæ, giving us the distance of whole classes of stars, discovering large numbers of spectroscopic binaries and checking up the motion of the solar system toward Lyra within a fraction of a mile per second. All told, photography has been the most potent adjunct in astronomical research, and it is impossible to predict the future with more powerful apparatus and photographic processes of higher sensitiveness. The field of research is almost boundless, and the possibilities practically without limit. What would Herschel have done with £100,000--and photography! CHAPTER XXII MOUNTAIN OBSERVATORIES The century that has elapsed since the time of Sir William Herschel, known as the father of the new or descriptive astronomy, has witnessed all the advances of the science that have been made possible by adopting the photographic method of making the record, instead of depending upon the human eye. Only one eye can be looking at the eyepiece at a time: the photograph can be studied by a thousand eyes. At mountain elevations telescopes are now extensively employed, and there the camera is of especial and additional value, because the photograph taken on the mountain can be brought down for the expert to study, at ease and in the comfort of a lower elevation. We shall next trace the movement that has led the astronomer to seek the summits of mountains for his observatories, and the photographer to follow him. Not only did the genius of Newton discover the law of universal gravitation, and make the first experiments in optics essential to the invention of the spectroscope, but he was the real originator also of the modern movement for the occupation of mountain elevations for astronomical observatories. His keen mind followed a ray of light all the way from its celestial source to the eye of the observer, and analyzed the causes of indistinct and imperfect vision. Endeavoring to improve on the telescope as Galileo and his followers had left it, he found such inherent difficulties in glass itself that he abandoned the refracting type of telescope for the reflector, to the construction of which he devoted many years. But he soon found out, what every astronomer and optician knew to their keen regret, that a telescope, no matter how perfectly the skill of the optician's hand may make it, cannot perform perfectly unless it has an optically perfect atmosphere to look through.