--------------+------+----+------+------+----+------+----+------+---- In the table on p. 212 the first column contains the ordinary trade names by which the various types of glass are known. These names, while somewhat arbitrary, indicate in a rough way the chemical nature of the glass concerned. Thus the word “flint” always implies a glass containing lead and therefore having a comparatively high refractive index and low value of ν, while the word “crown,” originally applied only to lime-silicate glasses, is now used for all glass having a high value of ν. In the next column of the table are given the refractive indices of the glasses, while the third column contains the values of ν. It will be seen that the glasses are arranged in descending order of magnitude in respect of this constant. An inspection of the figures in these two columns will reveal the fact that for the majority of the glasses contained in this table the value of ν decreases as the refractive index increases. The glasses which are an exception to this rule are indicated by an *. As a matter of fact this rule applied to all glasses that were known or were at all events commercially available prior to the modern advances in optical glass manufacture which were initiated by Abbé and Schott of Jena. It was Abbé’s insight into the requirements of optical instrument design that led him to realise the importance of overcoming this limitation in the ratio between the dispersive and refractive powers of glass. With the collaboration of Schott he succeeded in producing a whole series of previously unknown varieties of optical glass in which the relation between n_{D} and ν is not that of approximately simple inverse proportionality which holds for the older crown and flint-glasses. Most valuable and in many ways most typical of these new glasses are those known as the “barium crown” glasses, which combine the high refractive index of a light flint or even a dense flint-glass with the high ν value of an ordinary crown glass. It would lead too far into the subject of lens construction to explain in detail the possibility opened up to the optician by the use of these newer varieties of glass. We must content ourselves with pointing out that the great forward strides marked by the production of apochromatic microscope objectives, of anastigmatic photographic lenses, and the modern telescope objectives are all based upon the employment of these new optical media; and although optical glasses of these newer types are at the present time produced in the optical glass manufactories of France and England, in quality and quantity at least equal to the output of the Jena works themselves, these great optical achievements stand as a lasting monument to the pioneer work of Abbé and Schott in this field.