[Advaita-l] 'lens-grass' burning analogy

[V Subrahmanian]

Namaste

'lens-grass' burning analogy

I am looking for the reference of an analogy used in the texts to show that
'svarupa jnanam does not directly annihilate avidya, but when directed
through a vritti, it does the job'. The analogy is: sun directly does not
burn a blade of grass but when directed through a glass/lens, it burns.
This analogy is in some prakarana grantha or vyakhyana.  Any help in
locating the text is appreciated.

Thanks and regards
subrahmanian.v