Allusion has been made from time to time to Prescott's written memoranda and to his letters, which, indeed, were often very long and very frequent. It must not be thought that in writing these he had to make any use of his imperfect sight. The need of this had been obviated by an invention which he had first heard of in London during his visit there in 1816. It was a contrivance called "the noctograph," meant for the use of the blind. A frame like that of a slate was crossed by sixteen parallel wires fastened into the sides and holding down a sheet of blackened paper like the carbon paper now used in typewriters and copying-machines. Under this blackened paper was placed a sheet of plain white note-paper. A person using the noctograph wrote with a sort of stylus of ivory, agate, or some other hard substance upon the blackened paper, which conveyed the impression to the white paper underneath. Of course, the brass wires guided the writer's hand and kept the point of the stylus somewhere near the line.[8]
Of his noctograph Prescott made constant use. For composition he employed it almost altogether, seldom or never dictating to a scribe. Obviously, however, the instrument allowed no erasures or corrections to be made, and the writer must go straight forward with his task; since to go back and try to alter what had been once set down would make the whole illegible. Hence arose the necessity of what Irving once described as "pre-thinking,"--the determination not only of the content but of the actual form of the sentence before it should be written down. In this pre-thinking Prescott showed a power of memory and of visualisation that was really wonderful. To carry in his mind the whole of what had been read over to him in a session of several hours,--names, dates, facts, authorities,--and then to shape his narrative, sentence by sentence, before setting down a word, and, finally, to bear in mind the whole structure of each succeeding paragraph and the form in which they had been carefully built up--this was, indeed, an intellectual and literary achievement of an unusual character. Of course, such a power as this did not come of itself, but was slowly gained by persistent practice and unwearied effort. His personal memoranda show this: "Think closely," he writes, "gradually concentrating the circle of thought." And again: "Think continuously and closely before taking up my pen. Make corrections chiefly in my own mind." And still again: "Never take up my pen until I have travelled over the subject so often that I can write almost from memory."