Full Idea
One reason often alleged for the subjectivity of sense-data is that the appearance of a thing itself may change when we find it hard to suppose that the thing itself has changed - as when we shut our eyes, or screw them up to make things look double.
Gist of Idea
Sense-data may be subjective, if closing our eyes can change them
Source
Bertrand Russell (The Relation of Sense-Data to Physics [1914], §VIII)
Book Reference
Russell,Bertrand: 'Mysticism and Logic' [Unwin 1989], p.157
A Reaction
Russell firmly denies that they are subjective. These examples are also said to support to proposed existence of sense-data in the first place, since they show the gap between appearance and reality.