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Captain Abney and Major-General Festing. [May V6 T 



IV. " Intensity of Radiation through Turbid Media." By 

 Captain Abney, RE., F.R.S., and Major-General Festing,. 

 R.E. Received May 3, 1886. 



In the Bakerian Lecture for this year, which was delivered before 

 the Royal Society on March 4th, on Colour Photometry, we gave 

 incidentally the results of some measurements we had made of the 

 intensity of visible radiation which penetrated through a transparent 

 medium as compared with that which penetrated through the same 

 medium rendered turbid. We showed that the formula deduced by 

 Lord Rayleigh from the scattering of light by small particles* was 

 confirmed by our experiments. We thought, however, that the theory 

 might be more fully tested if a larger range of spectrum than that 

 to which we had confined ourselves were used, and at the same time 

 it would be more satisfactory if an instrument possessing no personal 

 equation could be utilised. Our thoughts naturally turned to the 

 thermopile, and more particularly to that form which we described in 

 a previous communication to the Royal Society (" Proc. Roy. Soc," 

 vol. 37, p. 157, 1884), since its delicacy was extremely great. In 

 the Bakerian Lecture the results we gave were obtained from water 

 rendered turbid, and it appeared to us that the same medium would 

 again answer our purpose — more especially if certain precautions 

 were taken. It will be in the recollection of the Society that we 

 have shown that water possesses very definite absorption-bands in 

 the infra-red of the spectrum ("Proc. Roy. Soc," vol. 35, p. 328, 

 1883), as also does alcohol. Now, as the turbidity of the water we 

 desired to produce was made by adding a dilute solution of mastic 

 dissolved in alcohol to the water, it was evident that in addition 

 to the water-spectrum we should also have superposed a faint 

 alcohol spectrum, if the addition of the latter was made in any 

 quantity. Had the spectrum of a definite thickness of pure and 

 transparent water been compared with one of the same thickness 

 of water rendered turbid by the mastic, it is evident that a dis- 

 crepancy might have arisen owing to alcohol being present in the 

 one case and not in the other. To avoid this difiiculty, when great 

 turbidity was to be produced, the amount of alcohol added with the 

 mastic was measured, and the same quantity of pure alcohol added 

 to the transparent water. By this plan the mixture of alcohol and 

 water was the same in the two cases, the only difference in the two 

 being that one had extremely fine particles of mastic suspended in it. 

 A reference to our paper (" Proc. Roy. Soc," vol. 35, p. 328, 1883) will 

 show that a small thickness of water cuts off nearly all the spectrum 



* "Phil. Mag.," vol. xli, p. 447, 1871. 



