Mr. T. Gaffield on the Action of Sunlight on Glass. 527 



greenish hue, and it is not so easily changed) , which changes to 

 a flesh-colour or a pinkish hue. I have accordingly taken these 

 two kinds for my under-glass experiments. Under each of se- 

 veral kinds, to be exposed from one to ten or twenty years, I 

 have placed pieces 4x2 inches 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 dif- 

 ferent series with each other, one can perceive the comparative 

 actinic power of each kind of glass, or rather their comparative 

 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 4 x 2-inch glass, taking in each piece at the end of the 

 year of its exposure. This series will show the diminishing or 

 increasing power of the glasses under which they have been ex- 

 posed 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. 



I have taken a piece of 4 x 18 of easily changing white plate, 

 painted with black paint two inches of each end to preserve the 

 original colour, and exposed the piece. At the end of the year 

 I shall paint over two inches more of the glass. At the end re- 

 spectively of two, three, four, six, and ten years I shall paint 

 over two inches more. At the end of this time, or a longer term, 

 I shall remove all the black paint, and on one light I shall have 

 all the grades of changed colour and shade produced by their 

 different lengths of exposure. I shall lay aside one piece of 

 4x 18 white plate, taken from the same sheet with the exposed 

 light, in order to compare the original with the changed specimen. 



I have painted and exposed, just in the same manner as above 

 described, a piece of 4 x 16 of easily changing Belgian sheet 

 glass. 



To show a speaking proof of the painting-power of the sun- 

 light, I have taken a piece of 4 x 6 Belgian sheet and covered 

 it with a thin plate of brass having the following letters cut out 

 of it: T. G., Jan. 1, 1867. I have taken another piece, 4x6 

 Belgian, and stuck on, with gum shellac, the two letters T. G. 

 After exposure of one year or more the removal of the brass 

 plate and letters will show in the former case rose- or purple- 

 coloured letters on a brownish -yellow ground, and in the latter 

 brownish-yellow letters on a rose- or purple-coloured ground. 



