The Day the Universe Changed - James Burke [33]
Al Hazen held that light came from sources of illumination such as candles or the sun and was then reflected off the object, carrying its image to the eye. Since light rays from many objects would thus enter the tiny pupil of the eye, it must in some way be able to focus them. The eye, therefore, had to be at the apex of a cone of light made up of visual rays from every part of the eye’s field of vision. Taking the analogy of a straight sword-cut going deeper than an oblique one, Al Hazen stated that the ray perpendicular to the eye was the strongest. He called it the ‘centric ray’.
Al Hazen’s theory had an extraordinary influence in the West among leading scholars such as Roger Bacon, John Pecham (Archbishop of Canterbury) and, in particular, Witelo, the Polish cleric and scholar, from whose writings Biagio da Parma had received it.
Biagio’s lectures were entitled ‘Questions on Perspective’, and as was the way at the time, Toscanelli had taken notes. He explained their content to Brunelleschi, whose initial interest may have been purely practical. The mathematics of perspective might make it possible to draw three-dimensional elevations of building plans for his clients, and this would enormously enhance his reputation as an architect. It may have been while working on such a plan that Brunelleschi carried out an experiment which was to prove one of the most fundamental in the history of Western thought.
At this time in the early fifteenth century, the glassworks on the island of Murano in the Venetian Lagoon had just started producing the new, flat, lead-back mirrors. Toscanelli showed Brunelleschi how this mirror exaggerated the perspective of the objects it reflected, because when you swivelled the mirror from the perpendicular position in front of the eye, what Al Hazen would have called the ‘centric’ position, the way in which objects diminished in size as they receded was very evident.
Brunelleschi put this idea into practice. He set up a mirror about six feet inside the main door of Florence Cathedral, facing outwards so that he could see the Baptistery, across the square, in the mirror. He then painted this reverse image on to a flat wooden tablet. Then he drilled a hole in the centre of the painting. Viewers were invited to look through the hole in the back of the painting while holding the mirror at arm’s length in front of the painting, so as to see it reflected in the mirror. As they were standing facing the Baptistery at the time, when the mirror was removed, they continued to see the Baptistery. Such was the accuracy with which Brunelleschi had done the painting that there was no discernible difference between the mirror-painting and the real thing.
This was the first example of perspective painting, and it must have had an extraordinary effect on people accustomed to the non-perspective representational styles of the period. Brunelleschi had chosen the Baptistery because its height, width and distance from the cathedral were almost exactly the same. Because of this the perspective ratio of all three dimensions was easy to reproduce - it was 1:1:1. By putting the peep-hole exactly where the eye level of the viewer would be while looking at the real Baptistery in the same position as he had chosen to paint, Brunelleschi had ensured that the painting faithfully showed all objects in their correct perspective to the viewer. The effect was that of looking through a window at a real scene.
A modern reproduction of what Brunelleschi’s perspective painting of the Baptistery must have looked like. The viewer saw the painting reflected in a mirror by looking through the hole in the centre of the door. It is probable that he covered the sky area with a mirror to reflect the clouds.
This was precisely the effect captured by