GR translation: milky panes beam beneficently down

[David Morris]

"Milky pane" are cloudy or whitewashed panes of glass, as in a
greenhouse.  "Beam" would be a beam of light.

On Wed, Dec 7, 2011 at 11:43 PM, Mike Jing
<gravitys.rainbow.cn at gmail.com> wrote:
> P7.28-31   Trusswork is pierced by daylight, milky panes beam
> beneficently down. How could there be a winter—even this one—gray
> enough to age this iron that can sing in the wind, or cloud these
> windows that open into another season, however falsely preserved?
>
> What are these "milky panes"?   And what is the meaning of the word "beam" here?