of glass, having average optical properties and showing dispersive powers roughly conforming to the law of inverse proportionality with refractive index which governs the older varieties of optical glass, are chemically stable substances, showing little tendency to undergo either chemical changes or to crystallise during cooling, the more extreme glasses exhibit these undesirable features to an increasing extent the more nearly the limit of our present range is approached. As the chemical composition of a glass is “forced” by the addition of special substances intended to affect its optical properties in an abnormal direction, so the chemical and physical stability of the glass is rapidly lessened. The more extreme glasses, in fact, behave as active chemical agents readily entering into reaction or combination even with relatively inert substances in their environment--they act vigorously upon the fire-clay vessels in which they are melted, and they are readily attacked by acids, moisture or even warm air, when in the finished condition, while many of them can only be prevented from assuming the condition of a crystalline (and opaque) agglomerate by being rapidly cooled through certain critical ranges of temperature. A limit to the possibility of production is set by these tendencies when they exceed a certain amount--a point being reached where it ceases to be practicable to overcome the tendency of the glass to self-destruction. On the lines of our present glasses, therefore, it does not appear hopeful to look for any considerable extension of the range of our optical media. On the other hand, as the known optical properties of transparent crystalline minerals show, a much greater range of optical constants would become available if it were possible to manufacture artificial mineral _crystals_ of sufficient size and purity for optical purposes, and the author believes that in this direction progress in optical materials is ultimately bound to lie[1]. [1] See a Paper by the present author on “Possible Directions of Progress in Optical Glass”--Proceedings of the Optical Convention, London, 1905.]