T. Gaffield on the action of Sunlight on Glass. 321 
a flesh color, or a pinkish hue. I have accordingly taken these 
two kinds, for my under glass experiments. Under each of seve- 
tal kinds, to be exposed from one to ten or twenty years, 
I have placed pieces 4X2 of the white plate. I shall take 
in one piece at the end of the first, second, third, fourth, sixth 
and tenth year. These six lights will show the increased action 
of the transmitted rays from year to year. By comparing the 
different series with each other, one can perceive the comparative 
_ actinic power of each kind of glass, or rather, their compara- 
_ live power of transmitting actinic rays. : 
____Another interesting under experiment is the following. I have 
_ Placed under one piece of each kind of glass exposed, a piece 
of easily changing glass, which I shall take in at the end of 
the year. 
_At the beginning respectively of the second, third, fourth, 
sixth and tenth years, I shall place under the same piece, 
another strip of 4X2 inch glass, taking in each piece at the end 
of the year of its exposure. This series will show the diminish- 
Ing or increasing power of the glasses under which they have 
n exposed, to transmit the actinic rays; in other words, will 
show whether exposure to the sun increases or diminishes the 
actinic power of the glasses exposed, and renders them better or 
Worse for photographic purposes. : 2 ; 
T have taken a piece of 4X18, of easily changing white plate, 
Painted with black paint two inches of each end, to preserve 
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, xposure of one year or more, the removal of the brass 
Plate and letters will show in the former ease, rose or pu 
Colored letters on a brownish yellow ground, and in the latter, 
brownish yellow letters on a rose or purple colored aioe 
_ Asimilar experiment a8 the above, I have commenced ' 
