672 PROCEEDINGS OF THE AMERICAN ACADEMY. 



In the course of the investigation here undertaken, for example, the 

 writer reached w. 1. 289 n^ through 5 mm. of Parra-Mantois "crown 

 extra Idger," nj) = 1.476. The existence of glasses of this order of 

 transparency is, as Zschimmer has pointed out, of importance in astro- 

 graphic and spectrographic work in which it may be desirable to take 

 advantage of all the rays which are transmitted by the atmosphere. 

 As is well known, the atmospheric absorption becomes exceedingly 

 strong below about w, 1. 310 /x/i, and extinguishes the solar spectrum at 

 about w. 1. 295 /x/*. Below this point, absorption is complete for the 

 whole thickness of the atmosphere, and as one nears w. 1. 200 fifi it is 

 practically complete, even for layers of a few cm. in thickness. This 

 is the region in which the atmospheric absorption is displayed by very 

 powerful ionization, which must therefore be a striking phenomenon in 

 the upper regions of the atmosphere, since the solar light, composed 

 as it is of well-known metallic radiations extending far into the ultra- 

 violet, fails to show these only in virtue of atmospheric absorption. 



The work of Kriiss and of Zschimmer was carried on by means of 

 spectrographs constructed of quartz, or quartz and fluorspar, and they 

 both used, as a source, the quartz mercury lamp, which is by far the 

 best available source for such investigations, inasmuch as when the 

 lamp is held at a uniform current it gives a wonderfully steady radia- 

 tion, which is not true of any other source rich in ultra-violet rays. 



From another standpoint, that of the investigation of glasses partic- 

 ularly adapted to cut off the ultra-violet, which are of interest to 

 the student of physiological optics, several investigators have recently 

 attacked the problem. In particular, important researches have been 

 published by Voege,^ Hallauer,^ and Schanz and Stockhausen.^ The 

 studies from the ophthalmological side have unfortunately mostly been 

 made with open electric arcs of various kinds, which are subject to so 

 considerable fluctuations, even under the most favorable circumstances, 

 as to make the comparison of diff"erent spectrograms, even when taken 

 in immediate succession, somewhat untrustworthy, although the dis- 

 tribution of intensity in a single spectrogram may be judged with 

 considerable confidence. 



In fact the quantitative value of spectrographic studies is somewhat 

 limited, even with the steadiest sources of radiation, on account of the 

 different performance of different makes of photographic plates and 

 the varpng action of developing and fixing baths. One can, however, 



^ The Illuminating Engineer, London, 2, 205, 543. 

 * Archiv. of Ophthalmology, 21, 1. 

 ^ Ztschr. f. Augenheilkunde, 23, 397. 



