Sentence:  The Celebrity, by arts unknown, induced Mrs. Judge Short and two other ladies to call at Mohair on an afternoon when Mr. Cooke was trying a trotter on the track....Their example was followed by others at a time when the master of Mohair was superintending in person the docking of some two-year-olds, and equally <WSD>invisible</WSD>. 
Meaning of the word marked with <WSD> tags:  Unable to be seen; out of sight; not visible.