Infrared prism spectrometry from 24 to 40 microns.

Some of the optical properties of the thallium halides have been known for a considerable time. The residual rays of some of these substances were measured by Rubens and ",Vartenberg 1 in 1914. The maxima of the refl ection bands were 9l.9 !L for TICI, 117 !L for TlBr, and 152 J.l. for TIl. These reflection maxima fall in thclongw"avelength region of the infrared. This fact indicated that the absorption of the material, ould not be intense for modcrate thicknesses to about 40 0(' 50 J.l.. Thallium iodide does not form cubic crystals and usually eAi sts a a powder. In order to use it for infrared studi es, a mixed crystal with TlBr from the fused mixture is grown . During the war such mixed crystals were grown in Jena. The material was called KRS5 and consisted of about 42 percent of TlBr and 58 percent of TIl. A mixed crystal of TICI and TlBr was called KRS6 and other materials that were used in the infrared region were also included in the KRS series. The KRS5 is highly resistant to water and was found useful on field instruments. Preliminary measurements in Germany showed that the TlBr-I crystal has a high index of refraction in the visible and the infrared regions and that in thickne..;ses of 2 mm it is transparent beyond 40J.l.. In 1945 plans were made at Fort Belvoir and at the Nation al Bureau of Standards for the growing of synthetic crystals of the thallium