Mr. T. Gaffield on the Action of Sunlight on Glass. 531 



give serious attention to the subject. It takes but a very slight 

 tinge of yellow to cut off twenty-five per cent, of the actinic 

 rays. I am forced to work under glass because of the protec- 

 tion from the wind and weather, but in doing so I sacrifice, in 

 any case, much of the light. I have to increase the time of 

 exposure proportionately. With the best of glass the loss from 

 reflexion and absorption is considerable. This may be fearfully 

 increased by the colour; and if that undergoes a continual 

 change for the worse, the state of things is exceedingly unplea- 

 sant. I was warned by a photographic friend in England to 

 take care of a certain kind of cheap French glass, made for 

 glass rooms, which turned yellow ; but I had no notion that 

 the evil was so general as you appear to have found it. The 

 subject is of such vital importance to photographers, that I 

 intend drawing the attention of that friend to what you have 

 done." 



It may seem strange that so long an experience in the window- 

 glass business had not at an earlier period drawn my attention 

 to the subject of this article. But my experience is not singular. 

 In conversation with many glaziers and glass-dealers I have 

 seldom found one who was aware of the great change of colour 

 effected by sunlight. Few have supposed that specimens were to 

 be found in any other windows in our city than those on Beacon 

 Street facing Boston Common and the south, and exposed to 

 the full force of the sun's rays. But having my attention now 

 particularly directed to the subject, I never pass a window with- 

 out detecting where any considerable change has been effected. 

 I have found them in all portions of the city, and most gene- 

 rally in those positions which face the east and south. The 

 colour is most easily detected when the glass has been ground 

 or enamelled, or where a white window-curtain forms a good 

 contrasting background. Many people suppose that the very 

 distinctly marked purple plate glass in Beacon Street was im- 

 ported of this colour, and that it is now no longer made, and hence 

 the reason why the windows looked like checkerboards when 

 broken lights were replaced by those of the usual light-greenish 

 colour of plate glass. 



On this point I have convinced many who had held contrary 

 views, by showing pieces from which the putty on the edges has 

 been removed, and displaying beneath the original colour. 

 When the putty covered the glass, the sun's rays could not reach 

 it, and the colour was unchanged. Such specimens are quite 

 interesting. I have had many conversations with old glaziers, 

 dealers and consumers of glass, and also with those who stain 

 and enamel it. I have thus gathered specimens of various kinds 

 and colours, and of differing lengths of exposure, from old win- 



