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The first point concerns the Balmer series of $$\text{H}_\text{ I}$$. The details of the Stark pattern vary from line to line in the series, but it happens that in every even member of the series, that is to say $$\text{H}\beta$$, $$\text{H}\delta$$, etc., there is no central, undisplaced Stark component. Under some circumstances, even if the hydrogen lines are broad and the Stark components are unresolved, this may result in a small dip at the top of an emission line, or a small bump at the bottom of an emission line. There are other effects (see the chapter on line profiles) that can result in a dip at the top of an emission line or a bump at the bottom of an absorption line, but if the cause is Stark splitting, this may be recognized in that it affects only the even members of the Balmer series, and not the odd members ($$\text{H}\alpha$$, $$\text{H}\gamma$$, etc.).