It was an addition, perhaps an afterthought, designed possibly to serve as a ground for surface-decoration, or to provide an alternative scheme for the completion of the dome.

Had Michelangelo really planned this innermost sheath, we could not credit him with the soaring sweep upwards of the mighty dome, its height and lightness, luminosity and space.

The roof that met the eye internally would have been considerably lower and tamer, superfluous in the construction of the church, and bearing no right relation to the external curves of the vaulting.

There would, moreover, have been a long dark funnel leading to the lantern.

Heath Wilson would then have been justified in certain critical conclusions which may here be stated in his own words.
"According to Michelangelo's idea, the cupola was formed of three vaults over each other.