Monochromatic Illumination.—Last year, in the October publication of your most excellent ‘Journal of Microscopical Science,’ which is forwarded to me by M. Baillière, of Paris, was published a letter from the Signor ab Count Francesco Castracane to the R. P. Secchi, about a certain mode of illumination, named a monochromatic one, and intended for the use of the microscope. One month after, a French review (the ‘Cosmos’) gave an account of a communication which had been made to the Institute about their mode of observing the microscopic phenomena. At last, in the first number of your journal for this present year, Mr. Barkas, of Newcastle-on-Tyne, has spoken also of this, Count Castracane’s new mode of illumination. On that matter I wrote to these gentlemen, and received from both of them most kind letters. We have since interchanged some specimens of diatoms, and to-day I am happy to number these two learned micrographers amongst my correspondents.