With a view to producing glasses which united in compound lenses should be color free, Rev. Vernon Harcourt, an English clergyman, in 1834 began experiments which he continued for twenty-five years. By using boron and titanium in addition to ordinary ingredients of glass, he produced lenses less troubled by color than any that had before been made. His labors, only in part successful, were in 1881 followed by those of Professor Ernst Abbe and Dr. Otto Schott at Jena. With resources provided by the Government of Prussia, these investigators were able to do more for the science and art of glass-making than all the workers who stood between them and the first melters of sand and soda. They immensely diversified the ingredients employed, carefully noting the behavior of each new glass, how much light it absorbed, how it behaved in damp air, what strength it had, how it retained its original qualities during months of keeping, and in particular how variously colored rays were distributed throughout its field of dispersion. As in the blending of new alloys it was found that many of these novel combinations were useless. Of the scores of new glasses produced some were extremely brittle, others were easily tarnished by air, or so soft as to refuse to be shaped as prisms or ground as lenses. A more systematic plan of experiment was therefore adopted: for the production of new glasses there were by degrees separately introduced in varied quantities, carefully measured, boron, phosphorus, lithium, magnesium, zinc, cadmium, barium, strontium, aluminium, berylium, iron, manganese, cerium, didymium, erbium, silver, mercury, thallium, bismuth, antimony, arsenic, molybdenum, niobium, tungsten, tin, titanium, fluorine, uranium. An early and cardinal discovery was that the relation between refraction and dispersion may be varied almost at will. For example, boron lengthens the red end of the spectrum relatively to the blue; while fluorine, potassium, and sodium have the opposite effect. With the distribution of the diverse hues of the spectrum thus brought under control, there were produced glasses which, when united as compound lenses, were almost perfectly color-free, rendering images with a new sharpness of definition. Yet more: in their unceasing round of experiments Professor Abbe and Dr. Schott came upon glass so little absorbent of light that combinations of much thickness intercepted only a small fraction of a beam; they were indeed almost perfectly transparent. This achievement is of great importance to the photographer, whose planar combination of six lenses may be four inches in thickness. At Jena the researchers are endeavoring to perfect another gift for the camera: they seek to produce glasses each transmitting but one color, for service in color-photography.