Agassiz was now threatened with a great misfortune. Already, in Paris, his eyes had begun to suffer from the strain of microscopic work. They now became seriously impaired; and for some months he was obliged to abate his activity, and to refrain even from writing a letter. During this time, while he was shut up in a darkened room, he practiced the study of fossils by touch alone, using even the tip of the tongue to feel out the impression, when the fingers were not sufficiently sensitive. He said he was sure at the time that he could bring himself in this way to such delicacy of touch that the loss of sight would not oblige him to abandon his work. After some months his eyes improved, and though at times threatened with a return of the same malady, he was able, throughout life, to use his eyes more uninterruptedly than most persons. His lectures, always delivered extemporaneously, do not seem to have been suspended for any length of time. The following letter from Agassiz to Humboldt is taken from a rough and incomplete draught, which was evidently put aside (perhaps on account of the trouble in his eyes), and only completed in the following May. Although imperfect, it explains Humboldt's answer, which is not only interesting in itself, but throws light on Agassiz's work at this period. AGASSIZ TO HUMBOLDT. NEUCHATEL, January 27, 1833. . . .A thousand thanks for your last most welcome letter. I can hardly tell you what pleasure it gave me, or how I am cheered and stimulated to new activity by intercourse with you on so intimate a footing. Since I wrote you, some things have become more clear to me, as, for instance, my purpose of publishing the "Fossil Fishes" here. Certain doubts remain in my mind, however, about which, as well as about other matters, I would ask your advice. Now that Cotta is dead, I cannot wait till I have made an arrangement with his successor. I therefore allow the "Fresh-Water Fishes" to lie by and drive on the others. Upon careful examination I have found, to my astonishment, that all necessary means for the publication of such a work are to be had here: two good lithographers and two printing establishments, both of which have excellent type. I have sent for Weber to engrave the plates, or draw them on stone; he will be here at the end of the month. Then I shall begin at once, and hope in May to send out the first number. The great difficulty remains now in the distribution of the numbers, and in finding a sufficient sale so that they may follow each other with regularity. I think it better to begin the publication as a whole than to send out an abridgment in advance. The species can be characterized only by good illustrations. A summary always requires farther demonstration, whereas, if I give the plates at once I can shorten the text and present the general results as an introduction to the first number. With twelve numbers, of twenty plates each, followed