Painters understood a lot about human vision long before scientists even invented themselves. This weekend, I look at two recognised types of visual illusion which were both discovered by artists long before anyone knew what an illusion is, and one piece of trickery which occurs in the optical projection used in painting. I have already looked at the Moon Illusion.

The Glare Effect

In this visual illusion, the brightness and colour of a target region are affected (enhanced) when it’s surrounded by suitable colour or luminance gradients. It’s much easier to see what I mean in the example below, in which four squares containing colour gradients make the central white square appear tinted, and significantly brighter than the white outside the squares. Amazingly, this wasn’t formally described until 1976, and it has since been realised that this effect was exploited to good effect by many painters from the Renaissance onwards.

The canonical example in painting is in Mathias Grünewald’s large Isenheim Altarpiece, painted in 1512-16.

The bright focus seen to the left of centre is in a panel known as the Concert of Angels and Nativity, shown in the detail below. Grünewald’s coloured gradient in luminance has the effect of making the crowned head stand out from within a brilliantly-lit oval.

In 1553-55 Tintoretto used a similar technique in the sky of his small masterpiece of Saint George and the Dragon. Above the saint, his quarry and the massive walls of a distant fortress is the figure of God, in a brilliant mandorla of light in the heavens. The artist used traces of vermilion and lead-tin yellow to tint this area.

The effect was well-known to Rembrandt, who used to to make the Hebrew letters stand out in his famous painting of Belshazzar’s Feast from around 1635-1638.

My last example is Velázquez’ last religious painting of The Coronation of the Virgin from about 1645. This is in amazingly good condition for its age, and has neither been relined nor retouched, allowing us to see the artist’s original work throughout. The white dove of the Holy Spirit is placed in an area which exploits the glare effect.

The Venus Effect

This arises in images in which the viewer sees a figure and their reflection (most typically of their face) in a mirror. There’s a strong tendency among viewers to believe that the figure is looking at their own reflection, and to completely misread the optical trickery employed by the artist in creating the image. This is thought to be the result of the viewer being able to see the figure’s reflection. Again, this is best understood by examples.

Although often illustrated by one of Titian’s paintings of Venus, for me the canonical example must be Velázquez’ Venus at Her Mirror, also known as The Toilet of Venus or the Rokeby Venus, from 1644-48.

It shows the goddess Venus, whose face is blurred in a false reflection in a mirror being held by her son Cupid. The theme was common, seen in paintings by Titian and Rubens, with Venus sat upright. Giorgione and others had posed her reclining and facing the viewer, making her pose here unusual. Most other paintings of Venus set her in a landscape: here she rests on luxurious even sensuous fabrics.

No matter how convincing her face might appear in the mirror, a moment spent placing yourself in the same position confirms that the image in the mirror is wholly imaginary, and optically incorrect.

There are several good early examples, seen in allegorical paintings of sight, for instance.

I’m not sure when Frans Floris’s Allegory of Sight was painted, but it was probably around 1550, making it an early and quite sophisticated entry to the subject. The face of its figure is shown reflected in the only appropriate optical instrument of the day: a simple mirror, which is angled carefully so as to project most of the face. Although only a small feature, that reflection looks fiendishly difficult, given the wildly different angle between the mirror and the picture plane. In this case, what is shown in the mirror is optically plausible.

This painting of Sight has been attributed to Abraham Janssens, and could date to any time between about 1590 and 1632. It appears to have been inspired by Floris’s Allegory of Sight, and the reflection of the woman’s face in the mirror doesn’t appear optically correct.

Mirror play using the Venus Effect has also appeared in paintings of the Old Testament stories of David and Bathsheba, and Susannah and the Elders.

Hans von Aachen’s David and Bathsheba of about 1612-15 introduces a figure standing behind Bathsheba, holding a mirror in front of her face with his outstretched left arm. A glance at that reflection says that something is seriously amiss: von Aachen has painted a reflection in which Bathsheba is looking to the left, although her face is actually looking to the right. No single plane mirror could ever achieve that optical impossibility.

Tintoretto’s Susannah and the Elders from about 1555 goes further with the mirror play. Susannah has been caught as she is drying her leg after bathing in the small pool beside her, looking at herself in a rectangular mirror, which is propped up against a rosy trellis in a secluded part of her garden. Unlike in other paintings of nudes, neither the image seen in the mirror nor the reflection on the water show anything more of Susannah.

More conventional use was made of the Venus effect during the nineteenth century.

JW Waterhouse’s Mariana in the South from about 1897 stands her in front of a full-length mirror which reveals her face to the viewer.

Laura Theresa Alma-Tadema’s A Knock at the Door, also from 1897, shows an attractive young woman checking that she is looking at her best in a mirror, before receiving a visitor. Once again it is the reflection which shows her face, and we’re struggling to be sure whether this is optically correct.

Of all the more modern artists, it was Pierre Bonnard who used mirrors most inventively, and explored the bounds of the Venus effect. Here I show just two of his earlier and more tantalising examples.

Bonnard’s unusual composition in this Interior from about 1905 doesn’t show the woman’s back in the mirror, but a chair placed quite deliberately in front of the mirror and Bonnard himself, not painting but sat at a table.

In his Woman Getting Dressed from 1906, it’s the mirror at the left which reveals the subject, who is sat beyond the right edge of the painting, getting dressed. Venus has here become a pile of discarded clothing.

Tomorrow’s article looks at the peculiar effects of three-dimensional projection, in foreshortening.

Reference

Arthur G Shapiro and Dejan Todorović (2017) The Oxford Compendium of Visual Illusions, Oxford UP. ISBN 978 0 19 979460 7.