Pope and Bishop Burgess

To what passage in Pope's writings does the conclusion of the following extract refer?[1] "Digammaticæ doctrinæ idem accidit. In his *Popius* eam in ludibrium vertit, &c. Sed eximius Poeta neque in veteribus suæ ipsius linguæ, nedum Græcæ monumentis versatus, tantum scilicet de antiqua illa litera vidit, quantum *de Shakespearii* SAGITTARIO." [1] 3d ed. of Dawes's *Mis. Critic*, p. xviii, note x.