748 VERHOEFF AND BELL. 



protection needed for artificial light sources was to reduce the light 

 from them at the working distance to substantially the range of the 

 sky spectrum. From these and other experiments they concluded 

 that the best protective glass should preferably reduce the spectrum 

 to approximately that of cloud or sky light. This again indicates 

 the use of neutral non selective absorbing glasses. Against this view 

 it may be properly objected that the eye in its evolution has rejected 

 the whole infra red and ultra violet as ineffective and in fact derives 

 very little useful illumination from the red at the one end and the 

 violet and blue at the other. The luminosity values of these portions 

 of the spectrum are very small and it may be added, fortunately very 

 small, else the chromatic aberration of the eye would make distinct 

 vision quite impossible. We are inclined, therefore, rather to the 

 view that such radiation as produces the maximum reqviired lumi- 

 nosity with the minimum energy access to the eye is best adapted to 

 protect the eye from any and all injuries which may be due to exces- 

 sive radiation. This indicates the use of glasses absorbing at both ends 

 of the spectrum so as to bring the strongest light in the region of great- 

 est luminosity, that is in the yellow green. As one of us has already 

 shown ^^ composite spectacles reducing the spectrum to a nearly 

 monochromatic stripe in this region actually enable one to view the 

 most powerful sources without discomfort while yet transmitting 

 enough light to permit writing or reading one's notes. The glasses 

 of Plate 7 in the deeper shades all show something of this character- 

 istic absorbing both ends of the spectrum and in so far represent 

 a slightly different type from the glasses which have preceded them. 

 Crookes found that suitable absorption at both ends of the spectrum 

 could not be obtained without encroaching somewhat on the visible 

 portion but rendered this encroachment rather inconspicuous by using 

 a heavy didymium glass which cuts out the yellow and leaves a 

 pinkish tinge. This, however, is not an objection in cases requiring 

 thorough protection unless the encroachment is so great as actually 

 to be inconvenient in seeing. Where, therefore, practical protection 

 against powerful sources of radiation is necessary, glasses meeting the 

 requirement of maximum luminosity with minimum energy present 

 material advantages. These advantages become practically inconse- 

 quential where the question is one of merely moderately reducing too 

 bright general light, and the choice between such special protective 

 media and ordinary neutral tint glass reverts again to a matter of 

 taste. 



