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 <WSD>proportional</WSD> to the incident light intensity . 
Meaning of the word marked with <WSD> tags:  properly related in size or degree or other measurable characteristics; usually followed by `to'