Sentence:  Thus my plight was evil indeed, for I had nothing now to burn to give me light, and knew that 'twas no use setting to grout till I could see to go about it. Moreover, the darkness was of that black kind that is never found beneath the open sky, no, not even on the darkest night, but lurks in close and covered places and <WSD>strains</WSD> the eyes in trying to see into it. 
Meaning of the word marked with <WSD> tags:  To exert or struggle (to do something), especially to stretch (one's senses, faculties etc.) beyond what is normal or comfortable.