I propose in this essay to sketch the principles on which some of the more recent wonders of telegraphic communication depend. I do not intend to describe at any length the actual details or construction of the various instruments employed. Precisely as the principles of spectroscopic analysis can be made clear to the general reader without the examination of the peculiarities of spectroscopic instruments, so can the methods and principles of telegraphic communication be understood without examining instrumental details. In fact, it may be questioned whether general explanations are not in such cases more useful than more detailed ones, seeing that these must of necessity be insufficient for a student who requires to know the subject practically in all its details, while they deter the general reader by technicalities in which he cannot be expected to take any interest. If it be asked, whether I myself, who undertake to explain the principles of certain methods of telegraphic communication, have examined _practically_ the actual instrumental working of these methods, I answer frankly that I have not done so. As some sort of proof, however, that without such practical familiarity with working details the principles of the construction of instruments may be thoroughly understood, I may remind the reader (see p. 96) that the first spectroscopic battery I ever looked through—one in which the dispersive power before obtained in such instruments had been practically doubled—was of my own invention, constructed (with a slight mechanical modification) by Mr. Browning, and applied at once successfully to the study of the sun by Mr. Huggins, in whose observatory I saw through this instrument the solar spectrum extended to a length which, could it all have been seen at once, would have equalled many feet.[27] On the other hand, it is possible to have a considerable practical experience of scientific instruments without sound knowledge of the principles of their construction; insomuch that instances have been known in which men who have effected important discoveries by the use of some scientific instrument, have afterwards obtained their first clear conception of the principles of its construction from a popular description.