Well, I landed a cache of old games (including Zork II
for the DEC Rainbow
 but the disk appears to be missing). One of them is Witness for DOS 2.0, and
 it even has the InvisiClues book -- but I don't have the marker! Does anyone
 have a spare InvisiClues marker somewhere they're not using or one they might
 be {willing, persuaded} to part with? 
AFAIR they can also be read well when using a _bright_
'warm' (yellowish) light, since the printed surface
is different from the unprinted - or use a blue (UV)
lamp, like available for bank note checking - here
the sourounding light schould be not to bright. Also
a damp/steamy athmosphere can reveal the content.
In versions you can avoide to change the book.
Gruss
H.
--
Ich denke, also bin ich, also gut
HRK