These are a few outstanding landmarks, peaks in the highlands of Greek science, and nothing has been said of their zoology or medicine. In all these cases it will be seen that the advance consisted in bringing varying instances under the same rule, in seeing unity in difference, in discovering the true link which held together the various elements in the complex of phenomena. That the Greek mind was apt in doing this is cognate to their idealizing turn in art. In their statues they show us the universal elements in human beauty; in their science, the true relations that are common to all triangles and all cones. Ptolemy's work in optics is a good example of the scientific mind at work.[81] The problem is the general relation which holds between the angles of incidence and of refraction when a ray passes from air into water or from air into glass. He groups a series of the angles with a close approximation to the truth, but just misses the perception which would have turned his excellent raw material into the finished product of science. His brick does not quite fit its place in the building. His formula _i_ (the angle of incidence) = _nr_ (the angle of refraction) only fits the case of very small angles for which the sine is negligible, though it had the deceptive advantage of including reflexion as one case of refraction. He did not pursue the argument and make his form completely general. Sin _i_ = _n_ sin _r_ escaped him, though he had all the trigonometry of Hipparchus behind him, and it was left for Snell and Descartes to take the simple but crucial step at the beginning of the seventeenth century.