Excerpt from Infra-Red Transmission and Refraction Data on Standard Lens and Prism Material The next best prism material is quartz, which is the most use ful in the region of the spectrum extending from the visible to a in the infra-red. Beyond this point a quartz prism is too opaque for practical work. From the standpoint of dispersion and transparency, a fluorite prism is the most useful in the region of 2 to 9 it. However, the material is difficult to obtain, and the next best substance is rock salt, which permits measurements to 14 a when using a 60 Degrees prism, and to 16 a when using a 30 Degrees prism. By inclosing the spectro meter '4 and by keeping the prism covered when not in use, the faces of a rock-salt prism are easily protected from moisture. About the Publisher Forgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.com This book is a reproduction of an important historical work. Forgotten Books uses state-of-the-art technology to digitally reconstruct the work, preserving the original format whilst repairing imperfections present in the aged copy. In rare cases, an imperfection in the original, such as a blemish or missing page, may be replicated in our edition. We do, however, repair the vast majority of imperfections successfully; any imperfections that remain are intentionally left to preserve the state of such historical works.