(See color Plate I) Reflections on a horizontal surface must align vertically with the object being reflected. In water with slight surface ripples, there is also blurring (appearing more in the vertical than horizontal direction because of the foreshortening of the ripple texture). The blurring does not interfere with the perceived reflectiveness of the water and the reflection is discounted — it is not seen as a pigment of the water surface but a reflection from it.

(See color Plate II) The reflection of the sun (moon?) in this painting by Tosa Mitsunobu, 1536, is offset from vertical alignment and should not even be visible on the lake when the sun is at such a low angle (the line of sight to the sun, reflecting off the water, should be blocked by the intervening hills). The vertical misalignment immediately stands out as ‘wrong’ in some ill defined way but the impossibility of the reflection is not so easily noticed, it requires additional mental geometry.

(See color Plate II) Even a hint of vertical organization helps organize a surface as a water reflection. The repeated daubs of color aligned vertically, trigger an interpretation of water even though the daubs are not aligned with anything in particular (Georges Braques).

(See color Plate III) Reflections do not have to match the reflected scene to be effective.

(See color Plate I) The Peanut-head Bug (Fulgora laternaria) from rain forests of Central and South America demonstrates the effectiveness of mimicked highlights despite their obvious inability to move appropriately as the observer moves.

(See color Plate I) When the reflections are skewed to break the vertical alignment, they are seen more as a texture of the surface than a reflection on it.

(See color Plate II) In contrast to the sensitivity to vertical alignment in water reflections, reflections for vertical reflecting surfaces are more complex. In a vertical mirror, the horizon and its reflection must be aligned horizontally but that does not hold for other horizontal contours unless the vertical surface is aligned with the direction of gaze toward the vanishing point and the horizontal contour is perpendicular to the reflecting surface. Note that on the left, the railing heads off at an odd angle in the window reflection (but the horizon does not). On the right, the image has been altered so that the horizontal railings should align horizontally with their reflections, but still do not. It is hard to judge which is correct (the left one). The artist may have arranged the bent reflection deliberately. If the door and direction of view had been arranged to make the reflection aligned (as they should be on the right), the collinearity of contours would be a cue that the glass was transparent, giving a direct view of the outdoors, rather than reflecting it. However, the bent railing breaks the visual system's rules for transparency, eliminating this interpretation; apparently, the bend does not break the visual system's rules for reflection, and this is what we see. Clever artist. Of course, the bends of the reflected railings on the right do break the rules of physics for both reflection and transparency.

(See color Plate III) Alexander Mosaic (100 BC). The fallen Persian sees his reflection in his shield.

(See color Plate III) Toilet of Venus, Rubens. Venus's eyes appear to be gazing at her own face but that is impossible as the rules of optics require that if we see her in the mirror, she sees us (or the painter). Note the highlights on the beveled edge of the mirror as a cue to the glass of the mirror.