In the ‘Quarterly Journal of Microscopical Science,’ vol. i, p. 34, an ingenious contrivance for registering the position of microscopic objects is described by Mr. Tyrrel; a modification of this, by Mr. Aymot, is given in a subsequent number (1. c., vol. i. p. 301) ; and a still better arrangement for the same purpose, suggested by Mr. Brodie and applied by Mr. Okeden to his microscope, is described at p. 166 of volume iii. of the same work. The last mentioned device can scarcely be improved upon for convenience; but there is one defect which is inherent to all these inventions, viz., that they are essentially selfish contrivances, of no use to any one but the owner of the particular instrument to which they may be attached.