BELL. — OPACITY OF GLASSES FOR THE ULTRA-VIOLET. G75 



much, owing perhaps to its less thickness, in the left-hand side of the 

 spectrum. It is, however, moderately transparent to the ultra-violet. 



Figure 5, Plate 1, is the result obtained from a very pale amethyst 

 glass in a thickness of 2.8 mm. This is a glass which has been con- 

 siderably used for protective spectacles in which great density may not 

 be desirable. It absorbs a trifle more of the extreme rays than do the 

 clear crown glasses, and conspicuously more from the violet to and 

 below w. 1. .360 fifi. The appearance of the spectrogram suggests some- 

 what selective absorption, but it is little more effective as regards 

 absorption in the ultra-violet generally than the common smoke glass. 



Figure 6, Plate 1, is a protective glass of a curious light yellowish- 

 pink hue, which has been considerably used under the name of Arundel 

 tint. It shows considerably stronger absorption than the amethyst 

 glass, especially for the shorter rays; the doublet at w. 1. 313 /^/ix is 

 completely wiped out. The next conspicuous line at w. 1. 334 nfi is 

 very greatly reduced in intensity, as is indeed the remainder of the 

 ultra-violet shown in this spectrogram. It also reduces the blue end 

 of the visible spectrum slightly, although its general tint is very light. 



Figure 7, Plate 1, is the quartz arc spectrum of Figure 1 as reduced 

 by a slip of Fieuzal glass 4 mm. thick. It completely obliterates the 

 whole ultra-violet region and the blank spectrogram is introduced here 

 merely for sake of contrast. It is a medium tint of yellowish-green 

 glass, cutting out much of the violet and blue and reducing considerably 

 the blue-green, although, since it retains all the rays of highest lumi- 

 nosity in the spectrum, it does not cut down the total light sufficiently 

 to serve as a protection against arc lamps or other extremely brilliant 

 lights. It is, however, a pleasant and effective glass where consider- 

 able reduction in intensity is not required. 



Figure 8, Plate 1, also a complete blank, was produced by a sheet of 

 ordinary commercial amber glass of medium tint and 2 mm. thick. Its 

 absorption of the ultra-violet is complete, and is strong in the violet, 

 blue, and blue-green. Its tint is due to letting through the red a little 

 more freely than does the Fieuzal glass just considered, which in gen- 

 eral properties it resembles rather closely. It is not sufficiently dense 

 to protect adequately against the dazzling effect of very brilliant 

 sources of light in the tint here examined, but is dense enough to serve 

 all ordinary purposes well. Its extinguishment of the ultra-violet, 

 however, is most complete, a property which it probably shares with 

 many other varieties of yellowish and orange-tinted glasses. A slip of 

 a reddish amber selenium glass, such as is sometimes used for railway 

 signals, 3.5 mm. thick, likewise cut off the whole ultra-violet region, 

 giving a blank spectrogram. This last-mentioned glass absorbed power- 



