The manufacture of optical glass is a costly and lengthy process. The chemicals used in its manufacture are selected with the greatest care; impurities must be guarded against for they would change the composition of the glass and in doing so alter its physical properties on which everything depends. The chemical substances are used either in the form of oxides, nitrates or carbonates, for the reason that they are easily decomposed by heat. To assist in the melting of the substances a few fragments of glass of similar composition are added. The crucibles, in which the chemical components are melted, are covered so that no fumes from the furnace may gain access to them, even the chemical composition of the crucibles is carefully tested that no impurities may contaminate the glass. No crucible is ever used more than once and only a single crucible is heated in each furnace, in order that the temperature may be regulated to a nicety. The actual manufacture is then begun after all these preliminaries have been attended to. A clean dry crucible is heated in a furnace--not the one in which the glass making is to take place--to a dull red heat. Then, with iron tongs, it is removed to the previously heated glass making furnace and the temperature is raised very gradually. The next stage sees the addition of the well mixed chemicals to the heated crucible, in small quantities at a time. When the full quantity has been added the crucible contains melted glass full of bubbles, some of them air bubbles released from the raw materials as they were added and some bubbles of gas given off from the chemicals as they act upon one another. The molten glass is then heated strongly so that it will become perfectly liquid and many of the bubbles will be driven off. To reach this stage may occupy anything from thirty-six to sixty hours and constant attention is necessary during the whole time.