the ideas of the colors themselves; for the ideas of greater or lesser degrees of refrangibility being applied to these words, and the blind man being instructed in what other respects they were found to agree or to disagree, it was as easy for him to reason upon the words as if he had been fully master of the ideas.  Indeed it must be owned he could make no new discoveries in the way of experiment.  He did nothing but what we do every day in common discourse.  When I wrote this last sentence, and used the words every day and common discourse, I had no images in my mind of any succession of time; nor of men in conference with each other; nor do I imagine that the reader will have any such ideas on reading it.  Neither when I spoke of red, or blue, and green, as well as refrangibility, had I these several colors, or the rays of light passing into a different medium, and there diverted from their course, painted before me in the way of images.  I know very well that the mind possesses a faculty of raising such images at pleasure; but then an act of the will is necessary to this; and in ordinary conversation or reading it is very rarely that any image at all is excited in the mind.  If I say, “I shall go to Italy next summer,” I am well understood.  Yet I believe nobody has by this painted in his imagination the exact figure of the speaker passing by land or by water, or both; sometimes on horseback, sometimes in a carriage:  with all the particulars of the journey.  Still less has he any idea of Italy, the country to which I proposed to go; or of the greenness of the fields, the ripening of the fruits, and the warmth of the air, with the change to this from a different season, which are the ideas for which the word summer is substituted; but least of all has he any image from the word next; for this word stands for the idea of many summers, with the exclusion of all but one:  and surely the man who says next summer has no images of such a succession, and such an exclusion.  In short, it is not only of those ideas which are commonly called abstract, and of which no image at all can be formed, but even of particular, real beings, that we converse without having any idea of them excited in the imagination; as will certainly appear on a diligent examination of our own minds.  Indeed, so little does poetry depend for its effect on the power of raising sensible images, that I am convinced it would lose a very considerable part of its energy, if this were the necessary result of all description.  Because that union of affecting words, which is the most powerful of all poetical instruments, would frequently lose its force along with its propriety and consistency, if the sensible images were always excited.  There is not, perhaps, in the whole AEneid a more grand and labored passage than the description of Vulcan’s cavern in Etna, and the works that are there carried on.  Virgil dwells particularly on the formation of the thunder which he describes unfinished under the hammers of the Cyclops.  But what are the principles of this extraordinary composition?