 As Descartes surely knew from experience, red is the last color of the primary rainbow (located in the uppermost section of the bow) and the first color of the secondary rainbow (located in the lowermost section of the bow). As he also must have known from experience, the red in the primary rainbow is much brighter than the red in the secondary rainbow. Consequently, Descartes’ observation that D appeared completely red and more brilliant than all other parts of the flask must have immediately struck him as significant and promising. He therefore proceeded to explore the relation between the rays of the sun, the position of his eyes, and the brightness of the red at D by varying the conditions, observing what changes and what remains the same in order to more precisely determine the relevant factors. The brightness of the red at D is not affected by placing the flask to the right or to the left of the observer, nor by the observer turning round the flask, so long as the angle DEM remains the same. When Descartes measures it, the angle DEM is 42º. Therefore, it is the 42º angle the eye makes with D and M at DEM alone that plays a role in the appearance of the brighter red at D. Having identified the measure of angle DEM, Descartes then varies the angle in order to determine what other changes, if any, occur. This enables him to correlate the decrease in the angle to the appearance of other colors in the flask: 