ACTION OF SUNLIGHT ON GLASS. 71 



"The fact of coloration," says Mr. Gaffield, " or change of color or 

 shade by sunlight being established, must we not transfer some of our 

 praise for the old artists in glass to the wonderful pencil of the bi'ight- 

 est luminary of the heavens, which, during the centuries, has noise- 

 lessly but unceasingly been at work, deepening and mellowing the 

 colors of all the windows of the venerable cathedrals of the world ? 

 We do not see to-day the glass as it was when it came from the ar- 

 tist's studio or the glass-factory. The dust of ages has accumulated 

 upon its surface, the corroding tooth of Time has eaten into it ; but 

 how often has the wonderful alchemy of the sunlight done more in 

 penetrating beneath the surface, and changing the shade or color of 

 the entire body of the glass ! " 



The cause of the interesting phenomena above stated has not yet 

 been explained, although not a few theories have been advanced to 

 account for it. The change in color has been variously attributed to 

 the presence of oxide of iron, to arsenic, to sulphur, and to oxide of 

 manganese, in the constituent materials of the glass. But the true 

 solution remains yet to be given. That the effect is not due to heat, 

 but solely to the actinic rays of the sun, is shown by the fact that no 

 change of color is produced in the glass when exposed to heat ; while, 

 on the contrary, after the discoloration has been produced by solar 

 light, the colors thus acquired disappear under the action of heat, and 

 the glass assumes its normal color. This process may be repeated 

 indefinitely, the change of color being produced by solar light, and 

 the original color restored by heat. It has also been shown that the 

 effect is not produced by air or moisture. 



Prominent among other interesting facts shown by these experi- 

 ments is the varying effect of sunlight during each season, and each 

 month of the year. The comparative actinic power of the rays during 

 each month is shown, at the end of the year, by the comparative depth 

 of yellow or purple color produced. The actinic effect increases from 

 January to July, and decreases after the latter month ; the greatest 

 effect is observed in the summer, and the least in winter ; in the spring 

 and autumn it is about equal, being midway between that of winter 

 and summer. 



The comparative power of different kinds of glass to transmit the 

 actinic rays was also tested. Of the colored glasses it was found that 

 blue transmitted the most, purple less, and red and orange the least ; 

 which shows the propriety of the preference given by photographers 

 to blue glass for skylights, because it transmits the blue rays, which 

 exert the most active power. Mr. Gaffield's observations have not 

 been confined to the glasses exposed on his own roof and window-sills ; 

 but he has been earnest in collecting, from various sources, specimens 

 of window-glass that have been exposed for greatly-varying periods. 

 By observing the portion of the pane exposed and that protected by 

 the sash and putty, a comparison is afforded between the original and 



