It's easy to generate a feminist reading of this picture, with its basic division of male/female reinforced by other divisions of clothed/naked, erect/abject, and direct view/reflected view. Notice that the prostitute is actually standing behind her customer but that he is still taking the active role as he looks at her modest body in the mirror, and then you can throw in the term "gaze."
But one of the distinctive characteristics of Brassaï's art is that it subordinates all merely social, merely human aesthetics, such as the ones addressed by feminist criticism, to the formal demands imposed on his images by their satisfyingly tyrannical orders of light and darkness. Because the pattern of this image demands that faces be absent, the faces are absent. So if we wonder what expression the man in this picture might have chosen from his wardrobe of faces, we'll have to consider another kind of power image. This one, for instance:
Click to enlarge.
I found this image at the photography site http://www.nocaptionneeded.com, where it advertised a symposium called "The Aesthetics of Catastrophe" at Northwestern on June 5, 2009. All I know about the image is the title embedded in its metadata: "1906 Earthquake Train." Presumably it is one of the little local records of a catastrophe still remembered by history at large: the earthquake that destroyed much of San Francisco. Presumably the man in the picture is happy that he survived. Certainly his contrapposto pose communicates the same relaxed swagger as, say, Walt Whitman's in the title page daguerreotype that substituted for the author's name in the first edition of Leaves of Grass.
Notice, too, the way the man's hand rests on the wheel of the capsized locomotive. It speaks the body language of a conqueror. As my friend Walter Creed says, people love steam engines because their machinery is all exterior. In them we see a body that seems alive and muscled.
And now this mighty machine, the nineteenth century's paradigmatic image of progress, lies overthrown. Altered now, like one of the handcuffed prisoners at Abu Ghraib with a pair of panties on his head, it is less than the man who condescends to it, less than the girl, less than the dog. So we might take the expression on the man's face as a smile of triumph. The human has survived, says the smile. No matter how hot and panting the steam engine was until the moment the earth shook it off and moved on to its next customer, it was only a woman after all.
But this picture of the smiling railroadman was taken in daylight, in brilliant California sunshine. Brassaï, of course, is the photographer of darkness, and the expressions on the two faces in his photograph will forever be on the dark side of the image. The Abu Ghraib photographs too were taken during the night shift. That historical truth may be telling us something somber: that some kinds of catastrophe may turn out to be harder to survive than the one that once upon a time befell an innocently naked locomotive and the innocently clothed man who now touches it with an affectionately chaste hand.