G80 PROCEEDINGS OF THE MIERICAN ACADEMY. 



too frequent association with perfervid commendation of somebody's 

 special variety of protective glass. If as a matter of precaution it be 

 thought desirable in certain cases to cut off the whole ultra-violet 

 region, there is a liberal choice of entirely effective glasses available. 

 For this purpose the common commercial amber glasses such as are 

 used for spectacles and signal lenses seem to be quite as eflfective as 

 those especially made for such use, the choice between them being 

 chiefly a matter of taste. The very lightest tints of all of them un- 

 doubtedly may transmit faint traces of the ultra-violet rays nearest 

 the visible spectrum, but when there is density enough materially to 

 soften the visible light these traces disappear. For protection against 

 powerful sources of light at short range only the deepest tints reduce 

 the visible light sufficiently, and these are particularly opaque to the 

 ultra-violet. 



