what does dimmesdale think he sees

From the text:

Throwing his eyes anxiously in the direction of the voice, he indistinctly beheld a form under the trees, clad in garments so sombre, and so little relieved from the gray twilight into which the clouded sky and the heavy foliage had darkened the noontide, that he knew not whether it were a woman or a shadow. It may be that his pathway through life was haunted thus by a spectre that had stolen out from among his thoughts.