Dear Stephen,

It's a pleasure to find a clearly and independently thinking person :)

SH> Thank you for your comments and lengthy set of quotations which corroborate
SH> my contention that nimitta is a mental product not a sense object.

Ven. Buddhaghosa explicitly writes:

"But it has neither colour nor shape; for if it had, it would be
cognizable by the eye, gross, susceptible of comprehension and stamped
with the three characteristics. But it is not like that. For it is
born only of preception. in one who has obtained concentration, being
a mere mode of appearance."

Ta~nca kho neva va.n.navanta.m, na sa.n.thaanavanta.m. Yadi hi ta.m
iidisa.m bhaveyya, cakkhuvi~n~neyya.m siyaa o.laarika.m
sammasanupaga.m tilakkha.nabbhaahata.m, na paneta.m taadisa.m.
Kevala~nhi samaadhilaabhino upa.t.thaanaakaaramatta.m
sa~n~najametanti.

Visuddhimagga IV.31 (126)

SH> If one overlooks this,
SH> then one can go astray both in one's understanding of teh text and also in
SH> one's meditational practice.

Yes, and unfortunately many earnest people reaped fruits of wrong
understanding of meditational practice. Hopefully deep and sincere
study of the original texts will help to straighten the path.

For example, in the case of meditation on four primary colours, there
is a widespread interpretation of nimitta as 'after-image' and
extending the nimitta as 'visualization'.

However from the deeper understanding of texts and meditation one can
comprehend that in this case the practitioner fishes out the
perceptual image (nimitta) of the given colour, and gradually learns
to control the process of apperception (sa~n~na), perceiving at will
everything in that colour. He sees that colour everywhere.

Best wishes,
Dimitry

P.S. Let me share with you the diagram of paticca-samuppada

http://dhamma.ru/lib/paticca.pdf or
http://users.i.com.ua/~sangha/dharma/lib/paticca.pdf

and invite you to the section of academic resources at Sadhu!
directory

http://dhamma.ru/sadhu/