As we have said before, the diamond is the foulest of gems, and is exceedingly liable to be injured by faults, such as are described in the technical terms of the jewellers as ashes, gray spots, rusty places, flaws, cavities, fissures, veins, feathers, foreign bodies, wavy and vitreous spots. Very few diamonds can stand the test of the microscope and be pronounced perfect. Still these microscopic faults are not to be considered in the commerce of the gem, but only in the study of its origin and nature. The jeweller may properly pronounce perfect the gem whose faults cannot be detected by the human eye. Even the magnificent Regent has one small foul speck in it, according to Jeffries; and Sir David Brewster found in the Koh-i-noor three specks, or rather cavities, in its central portion, which appeared to view in more or less distinctness according as the light reached them. After a diamond has been cut it is not easy to ascertain its degree of perfection without careful examination, and this occupies considerable time. And gems which appear at first glance to be pure and perfect are often found to be defective after examination. Babinet, of the French Institute, adopted the following method to study the effects of the diamond, and it was his intention to apply the test during his leisure moments to the principal diamonds in France; but other important labors diverted him from his purpose. He pierced a hole in a white card, a little larger than the diamond to be examined, and then passed a ray of sunlight or of the electric lamp through this hole. In the pathway of this ray, at a certain distance from the hole behind the card, he placed the diamond so that the ray of light fell upon the anterior surface of the stone. The rays reflected from this anterior surface, or, in other words, the table of the diamond, and those which pass through the stone are reflected back on the card, where they exhibit a white image of the table surrounded by small bands iridescent with the prismatic colors. By this simple method Babinet found that if the diamond had been well cut the colors were considerable in number, were well separated, and equally spread around the white reflection of the table. As each of these bands indicates one of the lustres of the stone, it is easy to estimate them both in number, quality, and symmetry. Therefore the observer can not only detect the errors of the cutting of the gem, but decide upon the form best adapted for the stone.