with great exertion and expense, completed one magnifying thirty diameters, which we now know to be the greatest power possible with the form of lenses that he used, viz., a double-convex lens for the object-glass and a double-concave lens for the eye-piece.
With such crude instruments as these, Galileo made his well-known discoveries, which, coming just when they did, proved of great importance in giving an additional impulse to the then rapidly awakening intellect of Europe.
Soon after the death of Galileo the telescope was further perfected by Huygens, who, in the first place, invented the form of eye-piece which still bears his name, and gives a large, flat field with very sharp definition. Many variations of form, but no improvement in the seeing quality of telescopic eye-pieces, have since been made, so that from this time all improvements in the telescope have been necessarily confined to the object-glass.
Huygens next enlarged the single-lens object-glass to its greatest possible power. His largest telescope had an object-glass five inches in diameter, and a focal length of one hundred and twenty feet; this enormous focal length being absolutely necessary to reduce the blurring effect of the prismatically colored fringes, as well as spherical aberration, to such moderate limits that a magnifying power of upward of two hundred diameters could be employed.
To have watched Huygens at work with this telescope must have been an amusing sight. Its great length precluded the use of a tube, and therefore an assistant was obliged to slide the object-glass up and down a vertical pole, one hundred feet high, by a cord, while Huygens pointed the eye-piece at the object-glass by sighting along a string connecting the two, meanwhile steadying himself by resting his elbows on a two-legged wooden horse. A more difficult and unsatisfactory contrivance to use can hardly be imagined, yet, with this telescope, in 1655, he discovered the rings of Saturn, and one of its satellites.
Newton, about this time, hastily concluded, from experiments of his own, that refraction without prismatic color was out of the question, and that the refracting telescope was incapable of further improvement; he therefore abandoned the study of the refracting telescope, and turned his attention to the construction of reflectors, and thus narrowly escaped making that most important discovery—the achromatic object-glass—which, only two years after his death, actually was made by Dollond, who, in 1757, constructed one two and a half inches in diameter, corrected both for prismatic color and spherical aberration.
From that day the power of the refracting telescope rapidly increased, and up to the present moment has only been limited by the ability of the glass-makers to furnish large pieces of optically perfect glass.
