From laboratory measurements, we know that a particular spectral line formed by

The source of the hydrogen is coming towards us.

Explanation:

The wavelength of the hydrogen is decreasing as it goes from 486.1 nm to 486.0 nm.

The wavelength of a wave is directly proportional to the distance between the observer and the object.

For example, when the observer is far from the source, the observer will observe a longer wavelength which corresponds to red color if it is in the visible region.

As the wavelength is decreasing that means that the hydrogen particle ( source may be a star) is coming towards the observer.

Thus, we can conclude that the source of the hydrogen is coming towards us.