I found a good definition of Newton's Rings: ’Newton’s rings usually form due to interference created by the reflection of light from a flat surface onto a spherical surface. In scanners, moisture on either the negative or on the glass surface (the tiny spherical droplets of moisture are responsible for the concentric rings in the pattern) can result in this problem.’
Professional photographers suggest to buy an “anti-Newton glass” to avoid this problem; also the DigitaLIZA scanning mask should work fine, avoiding the contact between the negative and the scanner’s glass. I found another tip that works, and that is putting the negative in the scanning mask back-to-front, with the natural “curve” of the negative (caused by the canisters) up … but I think that the images lose a lot of their definition. So I found an easier way to defeat this terrible enemy.
All you have to do is put something like a thin cloth between the upper and the lower part of the scanner, leaving a little space between the film and the glass. For better results in scanning, you should cut your negatives and leave them for some hours under something heavy, to make them as straight as possible … Et voilà! No more Newton’s Rings!
If you want to avoid the light to enter in the scanner because of the thickness you created, you can simply put a blanket (or a sweater, everything works well) over the scanner.