Diminishingly. The word has a limited supply and implies a reduction in importance, magnitude, quantity, or quality. It is a word used to describe a gradual or imperceptible process or change. Consider the following examples:

The sunlight dwindled and finally disappeared behind the clouds.

The crowd grew smaller and smaller as the hours went on.

The flowers were diminishing in size as the summer days went by.

The diminishment of the flowers' size is a gradual process that will eventually end. In the first example, the sunlight gradually disappeared and in the second example, the crowd gradually decreased in size.