517 



medium by passing through which the amount of light has diminished 



to ^th. 



The difference between the incident and transmitted light, i. e. that 

 lost in passing through the medium, is made up (I) of a portion re- 

 flected, and (2) of a portion absorbed or extinguished. We hare ex- 

 perimentally determined the values of the coefficient of reflexion , 

 and the coefficient of extinction a, for the glass plates used in our 

 cylinders. We found that 4*86 per cent, of the chemical rays, from 

 a flame of coal-gas, which fall perpendicularly on a surface of crown 

 glass, are lost by the first reflexion ; and that the amount of light 

 absorbed in our plates was so small as to fall within the limits of 

 observational errors. The value of p for the plates of glass employed 

 was found to be 0'0509. When the coefficient of reflexion for 

 glass p is known, the amount of light a transmitted by n plates is 



found from the formula 1 ,/ 2 ~^ 1 x =<* Hence the amount of light 



transmitted by two plates is 0*823. We have confirmed the accuracy of 

 the calculated result by direct experiment, and obtained a value 0*800, 

 or a mean of 0'811 as the coefficient of transmission of our plates. 



If all the transparent media have not the same coefficient of re- 

 flexion, the order in which the media are placed will affect the amount 

 of transmitted light. We have given an example of the mode in 

 which the calculation must in this case be made, in the determination 

 of the coefficient of extinction of water. We found that the amount 

 of light absorbed by a column of water 80 millimetres thick was in- 

 appreciable. According to the method here adopted, it is possible 

 to determine the coefficient of reflexion of all transparent fluids for 

 the chemical rays. We have only determined the coefficient of re- 

 flexion for American mica ; for the chemical rays of a coal-gas flame 

 p was found to be =0'1017. From the coefficient of reflexion, the 



refractive index (i) can be calculated from the equation p=f^^\ 



or i ;=. The refractive index for crown glass thus calculated 



1+ Vp 



from p= 0-0509 is found to be i= 1'583; the refractive index for 

 Fraunhofer's line II has been optically determined to be between 

 1-5466 and 1-5794 (Buff's Physik). 



Another important element in the investigation of photochemical 

 extinction is the law according to which the optical coefficient of 



