260 



Dr. G. W. Royston-Pigott. 



Observations, &c. (continued). 



Paris 1879. Supplement au Catalogue. 8vo. Paris 1880. 

 Pilote de la Manche : Cotes nord de France, Tome I. 8vo. 

 Paris 1880. And thirty Charts and Plans. 

 Rangoon : — Report on the Irrawady River. By R. Gordon. Pour 

 Parts, folio. Bangoon 1879-80, The Author. 



Righi (Augusto) Contribuzioni alia Teoria della Magnetizzazione 

 dell' Acciaio. 4to. Bologna 1880. The Author. 



Savery (Tho.) Navigation Improv'd : Or, the Art of Rowing Ships 

 of all Rates, in Calms, With a more easy, swift, and steady Motion 

 than Oars can. 4to. London 1698. [Reprint.] 



Mr. R. B. Prosser. 



Wartmann (Auguste-Henry) Recherches sur l'Enchondrone, son 

 Histologic et sa Genese. 8vo. Geneve et Bale 1880. The Author. 



Williamson (Benjamin), F.R.S. An Elementary Treatise on the Integral 

 Calculus. 8vo. London 1880. The Author. 



Zollner (Johann C. F.) Transcendental Physics. Translated by 

 C. C. Massey, 8vo. London 1880. Mr. W. H. Harrison. 



" Microscopical Researches in High Power Definition." By 

 G. W. Royston-Pigott, M.A., M.D. Cantab., F.R.S. Re- 

 ceived May 23, 1879. Read June 19. [The portion 

 between square brackets received November 20.] Part I. 



[Plates 3 and 4.] 



In the present communication it is intended to advert to the 

 subjects — 



(1.) Minimum visibility. 



(2.) The effects of excessive angular aperture in obliterating minute 

 molecular structure. 



(0.) Minute measurement and a micrometer gauge. 



(1.) Minimum Visibility. 



Spider-lines miniatured down to the fourteenth part of the hundred- 

 thousandth of an inch have been made distinctly visible to ordinary 

 good eyesight under proper microscopical manipulation. The question 

 then arose whether an actual thing so small as the millionth of an inch 

 could be descried in the microscopic field of view. 



According to recent researches, it would appear from formulae derived 

 from the undulatory theory of light, tha.t brilliant disks are developed 

 from points of light which vary inversely in diameter with the increase 



