'Burning Glasses' in Ancient India, another question

[vidya at cco.caltech.edu]

> Is anybody aware of textual evidence which would suggest that
> Indian authors of epistemological texts (in the first millenium A.D. or
> earlier) knew of the existence of such artificial devices,
> which would interfere with perception?

Many advaita works utilize a stock metaphor of a sphaTika crystal appearing
colored because of being viewed through a colored gem. By itself the sphaTika
is clear, but falsely attributing the color of the other gemstone to it, is
described as an error of perception. I am not aware if the example is used by
Sankara or sureSvara, however. 

S. Vidyasankar