Penetrating the roof at various points these cylinders would afford a passage for the outer air to the heated chamber within, while a certain quantity of light would be admitted at the same time. The danger arising from the rains could be avoided to a great extent by giving them a slightly oblique direction. To this very day the Turkish bath-houses over the whole of the Levant from Belgrade to Teheran, are almost universally lighted by these small circular openings, which are pierced in great numbers through the low domes, and closed with immovable glasses. Besides which we can point to similar arrangements in houses placed both by their date and character, far nearer to those of Assyria. The Sassanide monuments bear witness that many centuries after the destruction of Nineveh the custom of placing cylinders of terra-cotta in vaults was still practised. In spite of its small scale these circles may be distinguished in the woodcut of the Sarbistan palace which we have borrowed from Coste and Flandin (Fig. 54).[226] These same writers have ascertained that the architects of Chosroes and Noushirwan employed still another method of lighting the rooms over which they built their domes. They gave the latter what is called an "eye," about three feet in diameter, through which the daylight could fall vertically into the room beneath. This is the principle upon which the Pantheon of Agrippa is lighted; the only difference being one of proportion. In Persia, the diameter of the eye was always very small compared to that of the dome. If we are justified in our belief that the constructors of the Parthian and Sassanide palaces were no more than the perpetuators of systems invented by the architects of Nineveh and Babylon, the Assyrian domes also may very well have been opened at the summit in this fashion. In the bas-relief reproduced in our Fig. 42, the two small cupolas are surmounted with caps around a circular opening which must have admitted the light. Moreover, the elaborate system of drainage with which the substructure of an Assyrian palace was honeycombed would allow any rain water to run off as fast as such a hole would admit it.[227] Whatever may be thought of these conjectures, it is certain that the architects of Nineveh--while they did not neglect accessory sources of illumination--counted chiefly upon the doors to give their buildings a sufficient supply of light and air. As M. Place says, when we examine the plans of Sargon's palace at Khorsabad we are as much astonished at the size of the doorways as at the thickness of the walls.[228]