678 PROCEEDINGS OF THE AMERICAN ACADEMY. 



which the total thickness would be at least doubled for the same 

 aperture. 



Figure 3, Plate 2, shows the absorption of a slip of clear green signal 

 glass, 2.4 mm. .thick. This particular glass gives absorption in the 

 violet and in the red and orange, and transmits not over 15 per cent of 

 the total light. The absorption in the further ultra-violet is complete, 

 and even the group at w. 1. 365 fifi gets through enormously weakened. 

 The particular protective spectacles previously mentioned were made 

 of a layer of this glass and of the reddish amber selenium glass before 

 mentioned, of equal thickness. They completely suppressed the ultra- 

 violet, violet, blue, much of the green and also most of the red and 

 orange, leaving a nearly monochromatic strip in the most luminous 

 part of the spectrum. The resulting combination transmitted ample 

 light to enable one to read instruments and notes, or even to read a 

 newspaper in fairly strong light, but cut down the intensity to a point 

 that enabled one to look with comfort at short range into the quartz 

 lamp or the most powerful commercial flaming arcs. 



Figure 4, Plate 2, is the same amber glass as Figure 8, Plate 1, taken 

 with the stronger illumination. Two of the lines in the violet came 

 faintly through, but the ultra-violet was completely suppressed. 



Figure 5, Plate 2, is the same glass as in Figure 7, Plate 1. This as 

 before completely suppressed the ultra-violet and let through the two 

 violet lines very faintly. 



Figure 6, Plate 2, is the spectrum taken through a slip of No. 1 

 Euphos glass, 4 mm. thick. This is a light yellowish green in color, 

 suppressing the blue and violet somewhat less completely than the two 

 previous glasses, but like them blocking out the ultra-violet effectively. 

 None of the four glasses just described cuts down the intense light of 

 the quartz lamp or arc lamps, when viewed at short range, sufficiently 

 to avert ocular distress after a short exposure. For adequate protec- 

 tion against such sources considerably deeper shades of all of them are 

 necessary and are readily available. It must be borne in mind in this 

 connection that among arc lamp workers and operators of electric fur- 

 naces cases of the so-called ophthalmia electrica, which is chargeable 

 to the extreme ultra-violet rays, chiefly the abiotic radiations already 

 referred to, are rare compared with distressing symptoms due to the 

 luminous rays. Protective glasses, therefore, as light in tint as these 

 of Plate 2 are, as a matter of practical experience, of comparatively 

 little service although they suppress the ultra-violet quite completely. 

 Deeper shades of any of them may be effective. 



Figure 7, Plate 2, is an exposure of the bare quartz tube for 5 

 seconds. This spectrogram received therefore 1.66 per cent of the 



