36. Near the middle of the seventeenth century, the velocity of light was discovered by Roemer; and, in 1663, James Gregory, a celebrated Scotch mathematician, published the first proposal for a reflecting telescope. But, as he possessed no mechanical dexterity himself, and as he could find no workman capable of executing his designs, he never succeeded in carrying his conceptions into effect. This was reserved for Sir Isaac Newton; who, being remarkable for manual skill, executed two instruments of this kind, in the year 1672, on a plan, however, somewhat different from that proposed by Gregory. 37. In the course of the year 1666, the attention of Sir Isaac Newton was drawn to the phenomena of the refraction of light through the prism; and, having observed a certain surprising fact, he instituted a variety of experiments, by which he was brought to the conclusion, that light was not a homogeneous substance, but that it is composed of particles, which are capable of different degrees of refrangibility. 38. By the same experiments, he also proved, that the rays or particles of light differ from each other in exhibiting different colors, some producing the color red, others that of yellow, blue, &c. He applied his principles to the explanation of most of the phenomena of nature, where light and color are concerned; and almost every thing which we know upon these subjects, was laid open by his experiments. 39. The splendor of Sir Isaac Newton's discoveries obscures, in some measure, the merits of earlier and subsequent philosophers; yet several interesting discoveries in regard to light and color, as well as many important improvements of optical instruments, have been made since his time, although the light by which these have been achieved, was derived principally from his labors. [Illustration: GOLDBEATER.] THE GOLD-BEATER, AND THE JEWELLER. GOLD. 1. The metals most extensively employed in the arts are gold, silver, copper, lead, tin, and iron. These are sometimes found uncombined with any other substance, or combined only with each other; in either of these cases, they are said to be in a _native state_. But they are more frequently found united with some substances which, in a great measure, disguise their metallic qualities, or, in other words, in a state of _ore_. The mode of separating the metals from their ores, will be noticed in connexion with some of the trades in which they are prepared for, or practically applied in, the arts. 2. Gold is a metal of a yellow color, a characteristic by which it is distinguished from all other simple metallic bodies. As a representative of property, it has been used from time immemorial; and, before coinage was invented, it passed for money in its native state. In this form, gold is still current in some parts of Africa; and even in the Southern states of our own country, in the vicinity of the gold mines, the same practice, in a measure, prevails.