In "Upon Julia's Clothes," the speaker's descriptions of Julia's outfit get progressively weirder as the poem develops. At first clothes are just that—clothes—but then they liquefy (3), become a "vibration" (5), and finally are able to glitter (6). Silk is no longer silk by the end, or at least not any silk that is immediately recognizable. So the reality of Julia's outfit gets morphed into a surreal representation of the speaker's all too real feelings for our girl.
There's nothing real or definite about this girl or her outfit. Herrick's poem is all about how we see objects under the influence of emotions and in very strange ways.
The speaker of "Upon Julia's Clothes" is totally self-centered, bordering on solipsistic. Reality for him is all about what and how he sees the world, nothing else.