178 



Capt. Abney. On the Effect of the [Dec. 15, 



The first numbers of these pairs of figures show the result of 

 exposures ten times longer than the exposures shown by the second 

 numbers of the pairs. 



Silver chloride in collodion by whatever means prepared, and 

 whether exposed with an excess of silver nitrate, or an excess of 

 soluble chloride, gave figs. 40 and 41, the former being the result of 

 exposure ten times longer than that shown by the latter. The mode of 

 development had no effect on the spectrum developed. 



The washed paper gave on development the same result as that 

 shown for the direct action of light, viz., fig. 37. The mode of 

 development had no effect on the result. 



The washed paper subsequently treated with a solution of sodium 

 chloride and again washed, when exposed to the spectrum gave on 

 development with either gallic acid, and silver nitrate, or with ferrous 

 citro-oxalate, the same figure as that obtained by the direct action of 

 light, viz., fig. 38. 



When a brief preliminary exposure to white light was given to 

 either the paper or the different emulsions, fig. 46 was obtained on 

 development. On looking at figs. 35 to 46 it will be seen that in- 

 variably the maximum intensity is reached between H and/i. Accord- 

 ing to many authors the maximum is near G, whilst, according to others, 

 it is in the ultra-violet. I have carried out about 200 experiments 

 on the chloride with sunlight and with the electric light, and in no 

 case have I found it possible to alter the maximum. Of course if 

 candle or gas light be used as a source the maximum will be about Gr, 

 since the ultra-violet rays are almost absent with these. The idea 

 suggests itself that the prismatic arrangements employed may be at 

 fault ; in some cases where the most definite results have been 

 registered, a direct vision spectroscope has been utilised. I need 

 hardly say that snch an arrangement, from the very nature of the 

 apparatus, is unsuited for photo-spectroscopy. Such a spectroscope 

 transmits very few rays beyond H, and at H their intensity is much 

 diminished. In order to settle the matter to my own satisfaction, I 

 used a diffraction grating with the same results as those shown in the 

 figures under consideration. In a paper read before the British 

 Association in August last, I pointed out the great need of caution in 

 measuring daylight intensity by the chloride, and my subsequent 

 examination of the subject has more than ever confirmed me in my 

 opinion therein given. 



Methods of obtaining Mixtures of Silver Iodide and Bromide, Silver 

 Iodide and Chloride, 8fc. 



To test mixtures of the iodide and bromide, paper was prepared 

 by immersing it in a solution of potassium iodide and potassium 

 bromide, the proportion of each being so arranged that there should 



