Therefore, while in reflection one observes clear rings against a dark background, and there is thus good contrast and high visibility, with transmission one observes clear rings against a clear background and the phenomenon is poorly visible owing to a lack of contrast.
To realize this, it must be kept in mind that the reflection coefficient for glass with normal incidence is R=0.04 and therefore, neglecting absorption, the transmission coefficient is T=0.96.
Therefore, when observing with reflection, since the reflected beam has undergone a reflection and the transmitted beam has undergone a reflection and two transmissions, their intensities differ by a factor of T2 equal to about 0.92 and are approximately equal and the minima are approximately null; on the other hand, with transmission, since the generic transmitted beam has also undergone two reflections with respect to the previous one, their intensities differ by a factor of R2 equal to about 0.002 and the minima are quite different from zero.
For this reason, in the devices built especially for observation of Newton's rings with transmission, in order to have good visibility the surfaces of lens and underlying disk facing each other have a thin coat of silver in order to increase the reflection coefficient, which may even reach values above 0.9.
Obviously with such high reflection coefficient values the visibility of reflected rings is very poor.