The Mentor · The Story of the Lens · Page 5

opticians, working independently of each other, made the first achromatic lenses. These lenses eliminated to a large extent the disagreeable color fringes about the images produced by refractors. This was accomplished by using an object glass made of two lenses instead of one. A double convex lens of crown glass was cemented to a plano-concave lens of flint glass. Now, flint glass is considerably more dense than crown glass and has a greater power to disperse light into its component colors. And since the double convex crown glass converges the light while the plano-concave flint glass diverges it one lens neutralizes the color effect of the other and a colorless image results. The refractive power of the convex crown glass is greater than that of the concave flint glass, however, and therefore the light is still brought to focus and an image produced. Without this most important discovery no great refracting telescope would have been possible.

$Forty-inch refracting lens mounted$ FORTY-INCH REFRACTING LENS MOUNTED
Yerkes Observatory, Williams Bay, Wisconsin

The progress in the manufacture of optical glass toward the close of the last century has also contributed to this end. With these discoveries the only limitation to the building of refractors seems to be the casting of the large discs of glass. Reflecting Telescopes But long before the invention of the achromatic lens, Newton, despairing of success with the refractor, had built the first real reflector in 1670. The principles of this form of telescope had been previously explained and embodied in a crude instrument by Gregory, whose name is attached to one of the reflecting systems. The principal types of reflecting telescopes are illustrated on page 2. In each one a large concave mirror gathers the light and reflects it to a second mirror, plane, concave or convex. This second mirror reflects the light into an eyepiece which produces a magnified image of the object. The eyepiece carries two plano-convex lenses and is really a compound microscope. The reflecting mirror was entirely free from chromatic aberration (disturbances in the rays of light), so troublesome in the refracting telescope and many reflectors were constructed. The "leviathan" built by Lord Rosse in 1845 was six feet in diameter and the mirror was of speculum metal, an alloy of copper and tin. These mirrors are now made of glass with a thin film of silver deposited upon the front surface. As this film becomes tarnished it is removed with acid and the mirror resilvered. A famous grinder of mirrors and builder of telescopes was Sir William Herschel, who, in the eighteenth century, made many important discoveries, notably that of the planet Uranus.
On comparison of the two types of telescopes, we find that the refracting telescope suffers less loss of light, gives better definition and is less clumsy to manipulate than the reflector. Some of the great refractors are the 18-inch telescope of 1861 now at the Northwestern University, the 26-inch glass ground for the United States Naval Observatory in 1871, the 30-inch telescope erected at Pulkowa, Russia, in 1881, the big 36-inch Lick telescope completed in 1888, and finally the giant