The sentence is surely correct on its own, but I am thinking about the issue of shifts in number, an issue that arises not only on the sentence level but also on the paragraph level.
Does anyone know how to distinguish the unfavorable kinds of number shifts from the appropriate kinds?
Your confusion is reasonable. You're wondering when the numbers rules you've learned come into force and when they don't. To us, the paragraph sounds natural and correct, but since English isn't your first language, you're looking for a rule that explains why.
Notice that the first two sentences are about the instrument in a collective or plural sense:
Glass harps haven't disappeared. People still play them. You have number agreement there, right?
But in the third sentence, the focus shifts, and the number shifts, too. It would be just as correct to end that line with "these beautiful instruments" - or even "that beautiful instrument," meaning the particular instrument you're listening to.
Can all three options be correct? Yes, but the focus shifts with each. In saying "
this instrument" rather than "
that instrument," the writer is saying that if you hear one, you won't just be impressed by that one, particular instrument you're hearing. You'll appreciate glass harps in general.
Since a glass harp is an instrument, referring to it in the sense of an idealized, not-specific singular if fine - like
the dog is a wonderful species, or
the poem is Great Britain's greatest art form.
