DR. JAMES BOTTOMLEY ON COLOKIMETRY. 183 



c c 



log — + log — + &c. = qt (log A;, + log K + &c.) , 



log 





or cr/= -. — — — -=constant. 



l0g(A-i>^'z K) 



In my last paper I stated that the law of absorption of 

 light given by Herschel appears to have been obtained a 

 priori ; I have not found in his memoirs any experimental 

 confirmation of it. The form of the expression has a some- 

 what formidable appearance, inasmuch as it involves the 

 measurements of infinite varieties of light. But suppose 

 that in the formula l.ah^, h is the same for every species of 

 light ; then we may write T = ¥%a or T=/;'I, if I denote the 

 incident light. In such a case the emergent light Avill be 

 of the same nature as the incident lights and will differ only 

 in intensity. Suppose the incident light to be white^ the 

 emergent light will be a white of less intensity — that is^ 

 will be a grey approaching to blackness as the length of 

 the column increases. A fluid medium affecting white 

 light in this way we might call a soluble black ; and my 

 aim in seeking to obtain such a fluid was to apply it to the 

 confirmation of the law. In a previous note I stated that 

 I had tried to obtain such a body. What I got was not 

 wholly satisfactory; but I thought that with it I might 

 obtain some approximate results. The solution I used 

 consisted of 500 cub. c. of the previously mentioned fluid 

 with 500 cub. c. of distilled water. 



The mode in which I proposed to operate was as follows. 

 Take two white lights of different intensities, say W, and 

 Wj, and look at them through the liquid. Suppose the 

 lengths of the columns when equality of intensity is ob- 

 tained to be /, and t^, then 



