vol. x. No. 43); and, thirdly, by myself in damp earth of a fieldopposite my house, and also under decaying fungi in thewoods of Ercall Heath, Shropshire. Mecznikow (Bull. de1'Acad. Imp. d. Sciences de St. P�tersbourg, s�r. 5. vol. v.)has described a form of Land-Planaria which he discoveredin the Botanical Gardens at Giessen a few years ago, underthe name of Geodesmus bilineatus. This, I am certain, isquite distinct from the Planaria terrestris of Dug�s (Annalesdes Scien. Nat. vol. xx.), since found in England by at leastthree individuals. I still adhcre to my opinion that the Englishspecies, which is identical with the Fasciola terrestris of Millerand the Planaria terrestris of Dug�s, is a true Planaria, dif-fering from that genus in no essential anatomical particulars.Prof. Grube has a memoir" Ueber Landplanarien" (in Jahres-bericht der Schles. Gesellsch. f. vaterl. Cultur, 1866, pp. 61-64), but I have had no access to it. Would you kindly givein your pages an abstract of that memoir ? I remain, Gentlemen, Yours faithfully, W. HOUGIITON, M.A., F.L.S. Preston Rectory, Wellington, Salop. August 12, 1870. PROCEEDINGS OF LEARNED SOCIETIES. ROYAL SOCIETY. April 28, 1870.-Dr. William Allen Miller, Treasurer and Vice-President, in the Chair. "On an Aplanatic Searcher, and its effects in improving High-power Definition in the Microscope." By G. W. ROYSTON-PIGOTT, M.A., M.D. Cantab., M.R.C.P., F.R.A.S., F.C.P.S., formerly Fellow of St. Peter's College, Cambridge. The Aplanatic Searcher is intended to improve the penetration, amnplify magnifying-power, intensify definition, and raise the objec-tive somewhat further from its dangerous proximity to the delicate covering-glass indispensable to the observation of objects under very high powers. The inquiry into the practicability of improving the performanceof microscopic object-glasses of the very finest known quality wassuggested by an accidental resolution in 1862 ofthe Podura markingsinto black beads. This led te a search for the cause of defectivedefinition, if any existed. A variety of first-class objectives, fromthe ji to the ,, failed to show the beading, although most carefullyconstructed by Messrs. Powell and Lealand. Experiments having been instituted on the nature of the errors,it was found that the instrument required a better distribution of Ann. & Mag. N. Hist. Ser. 4. Vol. vi. 17Royal Society.257
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