1. THE plan of the theatre itself is to be constructed as follows. Having fixed upon the principal centre, draw a line of circumference equivalent to what is to be the perimeter at the bottom, and in it inscribe four equilateral triangles, at equal distances apart and touching the boundary line of the circle, as the astrologers do in a figure of the twelve signs of the zodiac, when they are making computations from the musical harmony of the stars. Taking that one of these triangles whose side is nearest to the scaena, let the front of the scaena be determined by the line where that side cuts off a segment of the circle (A-B), and draw, through the centre, a parallel line (C-D) set off from that position, to separate the platform of the stage from the space of the orchestra.