The first instance is that of Mr.Blacklock, a poet blind from his birth.

Few men blessed with the most perfect sight can describe visual objects with more spirit and justness than this blind man; which cannot possibly be attributed to his having a clearer conception of the things he describes than is common to other persons.
Mr.Spence, in an elegant preface which he has written to the works of this poet, reasons very ingeniously, and, I imagine, for the most part, very rightly, upon the cause of this extraordinary phenomenon; but I cannot altogether agree with him, that some improprieties in language and thought, which occur in these poems, have arisen from the blind poet's imperfect conception of visual objects, since such improprieties, and much greater, may be found in writers even of a higher class than Mr.Blacklock, and who, notwithstanding, possessed the faculty of seeing in its full perfection.

Here is a poet doubtless as much affected by his own descriptions as any that reads them can be; and yet he is affected with this strong enthusiasm by things of which he neither has, nor can possibly have, any idea further than that of a bare sound: and why may not those who read his works be affected in the same manner that he was; with as little of any real ideas of the things described?
The second instance is of Mr.Saunderson, professor of mathematics in the University of Cambridge.

This learned man had acquired great knowledge in natural philosophy, in astronomy, and whatever sciences depend upon mathematical skill.

What was the most extraordinary and the most to my purpose, he gave excellent lectures upon light and colors; and this man taught others the theory of those ideas which they had, and which he himself undoubtedly had not.