Sentence:  It <WSD>was</WSD> possible to make estimates of the quantum yield by observing the extent of reduction of a uranyl oxalate actinometer solution illuminated for a known time in a typical reaction cell and making appropriate conversions based on the differences in the absorption spectra of uranyl oxalate and of chlorine , and considering the spectral distribution of the light source . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)