The following pictures show this problem on an example of a Zeiss Standard Optovar. The lens elements are separating and this leads to the rainbow effect. I have slightly rotated the Optovar on the spot in same lighting between pictures and you can see that the inner edge of the rainbow rotates with the rotation of the whole instrument. This would not occur if it was due to defraction of light from the glass only.
Andy
Delamination in top lens
Delamination in bottom lens:
