Thank you very much for the commentary. It explained me a lot.
NvG> <The word ruupii: ruupa which is the condition for the arising of
NvG> ruupajhaana, namely the ruupa in the body such as hair, etc. which is
NvG> internal. It is called ruupii: because the bhikkhu has ruupa. The words
NvG> ruupaani passatiiti: the bhikkhu sees the ruupa, he sees the ruupa of the
NvG> nilakasina (kasina of blue), etc. outside, by the eye of wisdom (~naa.na
NvG> cakkhu). By these words he explains the attainment of jhaana by the kasinas
NvG> which have their bases (vatthus) inside and outside. >
Such interpretation (elaborated in the Nyanatiloka's "Buddhist Dictionary"
in the Abhibhaayatana article) involves a 'spot on one's body':
(1) "Perceiving (blue..., red..., yellow..., white) forms on
one's own body, one sees forms externally small ones, beautiful
or ugly; and in mastering these one understands: 'I know, I
understand.' This is the first stage of mastery.
(2) "Perceiving forms on one's own body, one sees forms
externally, large ones .... This is the second stage of mastery.
(3) "Not perceiving forms on one's own body, one sees forms
externally, small ones .... This is the third stage of mastery.
(4) "Not perceiving forms on one's own body, one sees forms
externally, large ones .... This is the fourth stage of mastery.
(5) "Not perceiving forms on one's own body, one sees forms
externally, blue forms, forms of blue color, blue appearance,
blue lustre, and mastering these one understands: 'I know, I
understand. This is the fifth stage of mastery."
(6-8) The same is repeated with yellow, red and white forms.
As preparatory kasina-object for the 1st and 2nd exercise one
should choose on one's own body a small or a large spot,
beautiful or ugly, and thereon one should concentrate one's full
undivided attention, so that this object after a while reappears
as mental reflex or image (nimitta, q.v.) and, as it were, as
something external. Such an exercise, though appearing quite
mechanical, if properly carried out will bring about a high
degree of mental concentration and entrance into the 4
absorptions (jhana, q.v.). In the 3rd and 4th exercises the monk
by an external kasina-object gains the mental reflexes and
absorptions. As objects of the remaining exercises, perfectly
clear and radiant colors should be chosen, flowers, cloth, etc.
Unfortunately this interpretation (ajjhatta.m - on one's own body???,
aruupa - not forms???) does not make much sense, and is not
consistent at least with Vimuttimagga and Visuddhimagga.