Musar for Nedarim 39:11

You must know that every sin is brought about by an antecedent and a cause. In this case, too, there is an antecedent and a cause that bring a man to gaze at forbidden women. The first cause is the gazing upon impure things until the eye is sated. Although it is permitted to look at the strange creatures from foreign lands, for which the Sages even instituted a blessing, “Blessed is He who diversifies the creatures,” nevertheless, one should not sate the eye with gazing at them but only cast upon them a passing glance. For the eye is comprised of four colors (the white, the dark rim around the iris, the color of the iris and the black of the pupil) corresponding to the four letters of the Divine name (Tikkunim 70, 128a). When a man casts his gaze upon impure creatures he draws upon himself the unclean spirit that hovers over them. This later causes him to gaze upon worse things, which then bring him to sin. For this reason the Sages warned that a man should not even gaze at his own wife when she is a niddah (menstruating woman) because defilement hovers over a woman while she is having her menstrual flow and through his gaze he draws it to him, causing it to adhere to his eyes. The proof to this is that when a niddah stares at a new mirror her vision makes a stain upon the glass that can never be removed. For the same reason the Sages forbade gazing at the face of en evildoer. Instead a man should accustom himself to looking upon holy things. That way he draws holiness upon himself, imbuing the four colors of his eyes with great illumination.