Sentence:  The last column shows the rate of exchange that would have been observed at a relative intensity of 4 ( 14.7 cm. distance ) calculated on the assumptions that the incident light intensity is inversely proportional to the square of the distance of the lamp from the cell and that the rate is directly proportional to the <WSD>incident</WSD> light intensity . 
Meaning of the word marked with <WSD> tags:  falling or striking of light rays on something