Secondary qualities are properties that a thing appears topossess for certain observers of this thing. On reflection, however, secondaryqualities turn out to be ways in which certain observers are affected by thething in question. Colors are an example. Colors are not genuine propertiesinherent in things but rather ways in which human beings with normal eyesightare affected by certain things they encounter. Secondary qualities are thus to be explainedby reference to both: the object with its “primary” qualities and the perceptual apparatus of a specificobserver of this object.
The doctrine of secondary qualities brings with it thethought of the object as it is apart from whatever qualities it merely appearsto possess for certain observers. You can call this the thing it itself, thething considered apart from its merely apparent, observer-dependent properties.So the two expressions you query are not at all synonymous but rathercorrelative: “thing in itself” refers to an object as it is apart from whatever(secondary) qualities it possesses only for some or all of its observers.
Locke thought that paradigm primary properties of things aretheir geometrical shape and location in space. He took these properties to beinherent in things themselves, and thus to be independent of any and everyobserver – in Kant’s phrase, Locke considered the physical objects analyzed byphysics to be things in themselves. Kant, however, disagrees with thisassessment. His hypothesis is that even the spatial and temporal properties ofobjects are observer-dependent, that these are properties that physical objectsmerely appear to possess when we encounter them with our human sensibility. On Kant’saccount, the concept of a thing in itself refers then to things considered apartfrom their secondary qualities and fromtheir spatial and temporal features.