Full Idea
The ideas of primary qualities of bodies are resemblances of them, and their pattern do really exist in the bodies themselves; but the ideas produced in us by secondary qualities have no resemblance to them at all.
Gist of Idea
Ideas of primary qualities resemble their objects, but those of secondary qualities don't
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 2.08.15)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.137
A Reaction
I think this is exactly right. More that one sense can reinforce a primary quality, because there is a 'pattern' to be detected in various ways. That things look square is explained by their squareness; things looking red is just very weird.