162 



Captain Abney on the Opacity of the 



Plates were exposed on, this object, the negatives being ob- 

 tained by the ordinary wet process, with simply iodized collo- 

 dion, an 8-per-cent. nitrate-of-silver bath, and 4-per-cent. iron 

 developer. The strength of the developer was afterwards varied; 

 but for the purposes of these experiments any variation was ex- 

 cluded. Other negatives were obtained on dry plates made with 

 bromized collodion, a 16-per-cent. nitrate-of-silver bath, albu- 

 men preservative (washed off, after application, as far as possible), 

 and alkaline development of one particular strength. By alka- 

 line development, as is well known, the bromide of silver is re- 

 duced to metallic (or oxide of) silver in situ, no free nitrate of 

 silver being applied to the image during development. The 

 opacity of the image obtained by this method is particularly 

 adapted for giving the necessary means of measuring the action 

 of any relative intensities of light acting on the silver for any time. 



In order to determine the relative opacities of the image, it 

 was necessary to obtain some standard scale with which to mea- 

 sure. The ordinary methods were tried without success, the 

 image being " matt," or only translucent. Failure with them 

 was inevitable. After various experiments with coloured gela- 

 tine wedges, I determined to use coloured glass wedges, and, 

 owing to the kindness of Mr. Browning, obtained three smoke- 

 coloured ones, corrected for refraction by crown glass. These 

 in varying combinations have given. me every thing that could be 

 desired. The mounting I adopted for them is as follows. 



A is the wedge in position, B a space in the frame E, in which 

 any glass whose opacity is to be measured is placed, C a slit, 

 and D a fixed scale dividing the wedge into arbitrary divisions. 

 In actual use the whole of the frame was glazed with finely 

 ground glass, the slit being next to it, and the wedge against 

 that again. When measurements of opacity were taken, the 

 glass to be tested was placed in B and a light placed at a known 

 distance behind the slit. Great care was taken to ensure the 

 equal illumination of C. The length of the wedges are severally 

 6-5 inches. They do not give a zero of absorption at their thin 



