The Various Forms of Telescopes.

THEIR CONSTRUCTION AND ADVANTAGES.

ROM the time of Galileo the art of constructing telescopes has made great progress, each improvement marking an epoch in modern astronomical history. Besides inventing the telescopes which bear their names, the great Italian Astronomer and his successors did all they could to improve the forms already in use. For example, that form of the telescope, which in the hands of Galileo startled the world by the discovery of other worlds than ours, is at the present day represented by the very convenient and portable, though not powerful, instrument called the opera-glass. Powerful refracting telescopes differ from the Galilean in having a convex eye lens instead of a concave. If we look through a convex lens, or upon a concave reflecting surface, we shall see, if the eye is within the focus, an erect picture of surrounding objects which will enlarge as the eye is withdrawn until the focal point is reached: past this point the picture will appear inverted, and will now diminish as the eye is removed. It will therefore be obvious that if our eye-piece is without the focus we shall see an inverted picture. As it is necessary that a convex eye lens should be without the focus of the object-glass, in order to obtain distinct vision, the inverted picture must be re-inverted by extra lenses when we desire to view terrestrial objects, but this will be of no consequence in celestial observation. Before the invention of achromatic object-glasses, telescopes had to be made of most extravagant focal lengths, in order that the aberrations of the object-glass should be minimized. With these lengthy telescopes Huyghens was associated, and the eye-piece bearing his name was then first usedv He thus discovered how to correct an eye-piece, but failed to improve the object-glass. The introduction of achromatic glasses composed of lenses of different densities brings us down to modern times, and the names of Brewster, Dollond and Ramsden.

It was early noticed that these very long focus object-glasses were not only very awkward instruments to manage, but also that no length could ensure a colourless image, and therefore scientific men of those days turned their attention to reflecting telescopes which with the Huyghenian eye-piece showed objects free from colour.

Of reflecting telescopes, the earliest form was that devised by Gregory, but he does not appear to have ever made one. In this form the focal point is passed before the rays from the concave speculum fall upon a small concave mirror from which they are reflected to the eye-piece.

---

Le texte affiché peut comporter un certain nombre d'erreurs. En effet, le mode texte de ce document a été généré de façon automatique par un programme de reconnaissance optique de caractères (OCR). Le taux de reconnaissance estimé pour cette page est de 99,47 %.

La langue de reconnaissance de l'OCR est l'Anglais.

---