Vision and Definition under the Microscope. 163 



microscopical image as shown under low powers, and we are 

 indebted to Mr. T. H. Court and Mr. George H. Gabb for the loan 

 of interesting and rare types wliich were not in the Society's 

 collection. No doubt several other models could have been pro- 

 fitably included, but they have been omitted because, as is 

 unfortunately often the case, contemporary objectives were not 

 a\'ailable for use with the instruments. In order to facilitate 

 the comparison of the image in the different instruments we 

 have in every case exhibited the same organism, the common 

 Foraminifer Polystomella striato-punctata (Fichtel & Moll). We 

 have chosen this, not only because it is a suitable object for 

 medium powers, but also because it was one of the first Ehizopoda 

 ever figured,* appearing in the " Micrographia " of Hooke in 1665,t 

 and in the Letters of Professor Plimmer's " Immortal Beadle," % 

 Antony van Leeuwenhoek,§ the former " from small sand," the 

 latter "from the stomach of a shrimp." The slides have been 

 mounted, by Mr. Arthur Earland, in various methods contemporary 

 with the Microscopes employed. 



Class I. — The Early Type of Simple Microscope. 



1. Antony van Leeuivenhoek (c. 1673). 



This is a copy made for Sir Frank Crisp from that in his 

 possession, and presented to the Society by hhii, the original being 

 in the Zoological Laboratory at the University of Utrecht. (See 

 Journ. R.M.S., 1914, p. 105.) Two thin metal plates are fastened 

 together by rivets. Between these plates a very small double 

 convex glass lens is mounted between two concavities provided 

 with minute apertures. The object is held in front of the lens on 

 the point of a short pin, the other end of which screws into a 

 small block or stage of brass, which is riveted on the end of a 

 long coarse-threaded screw acting through a socket angle-piece 

 attached behind the lower end of the plates. It is with such 

 instruments, of the rudest kind mechanically, that Leeuwenhoek 

 astonished the world with his discoveries of Infusoria, Bacteria, 

 and other microscopic forms of life. 



Described and figured in Mayall's Cantor Lectures, 1885, 

 p. 20, in Journ. R.M.S., 1886, pp. 1047-9; 1909, p. 652, and 

 1914, p. 105 ; and Singer, Proc. Roy. Soc. Med., vii. 1914, p. 252. 



♦ L. C. Miall, Op. cit.,p. 140. 



t R. Hooke, " Micrographia," etc., London, 1665, p. 80, pL v. fig. 10. 



X H. G. Plimmer, '^ Bedellusimmorialis," Journ. R. Micr. Soc, 1913, p. 121. 



§ A. van Leeuwenhoek, "Sevende Vervolg der Brieven, Waar in gehandelt 

 werd van veele Opmerkens en verwonderenswaardige Natuurs-Geheimen," Delft, 

 1702, p. 195, pi. (opp. p. 191), fig. 7, a, b, c, " Vertoont sen van de Slak-hoorntjens 

 diebij mij uit de Maag van een Garnaad is geuomen." 



