ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 213 



Liquids, n ^ I'll to 1 " <S7.— Sulphur and the iodides of tin, arsenic i^ 

 and antimony are dissolved in methylen iodide. 



Liquids, n = 1*74 to 2 "28. — Arsenic trisulphide is dissolved in 

 methylen iodide, and the solution standardized by means of a gonio- 

 meter or a spectrometer and a prism. 



Resln-Jikp Substances, n = 1-68 to 2*10. — ^The triodides of arsenic 

 and antimony are dissolved in piperin. 



Mixtures of Amorphous Suljjhur and Arsenic Trisrdphide, n = 'l'\ 

 to 2 "6. — These mixtures are much lighter-coloured than corresponding 

 ones of sulphur and selenium, but they are less easily standardized and 

 manipulated. They should be nsed only in cases requiring greater tran- 

 sparency than the sulphur-selenium mixtures. 



Permanent Standard Resinous Media, n = 1'546 to 1-682. — Any 

 proportions of piperine and resin form a homogeneous fusion, which 

 cools to a transparent resinous mass. 



Permanent Standard Resinous Media, n = 1*510 to 1"516. — This 

 series is prepared from resin and camplior. 



Permanent Standard Fluids, n ^ 1' 487 to 1 • 683. — These are formed 

 from certain organic solids, which form eutectic mixtures melting much 

 below ordinary temperature. 



Resolving Power of the Microscope.* — A discussion on the above 

 subject between A. S. Percival and E. Leitz, extending over several 

 numbers of the Lancet, resulted in reaching the following points of 

 agreement. 1. That the smallest distance between two adjacent bars 



of a grating which can be resolved is given by ^ = . 2. That the 



a 



formula d = 0*61 - is correct when a telescope is employed to resolve 

 the distance between two stars, as in the case of doubles. 



C6) Miscellaneous. 



Quekett Microscopical Club. — The 486th Ordinary Meeting was 

 held on January 28, 1913, the President, Prof. A. Dendy, F.R.S., in 

 the Chair. W.M. Bale, F. P.M. S. : -'Notes on some of the Discoid 

 Diatoms." H. Whitehead, B.Sc. : A paper on " British Fresh-water 

 Rhabdocoelida (Planarians)." E. M. Nelson, F.R.M.S., in a " Note on 

 Pleurosigma angulaium^^ using a Leitz yV apochromatic of N. A. 1 * 4, 

 has unmistakably seen in this and allied forms that the apertures in 

 the lower membrane are below the intercostals of the upper and are not 

 " eye-spotted," that is, that the aperture in the lower membrane is 

 directly below the aperture in the upper membrane, as was formerly 

 held to be the case. C. F. Rousselet, F.R.M.S., read a note on " Some 

 Rotifers from Devil's Lake, North Dakota, U.S.A." 



The 47th Annual General Meeting was held on February 25, 1913. 

 Prof. A. Dendy, F.R.S., dehvered the annual address, and dealt with 

 " By-Products of Organic Evolution." March 25. Messrs. Heron- Allen 

 and Earland on " Some Foraminifera from the southern area of the North 

 Sea." The investigation of six dredgings — three from near the Great 



* Lancet, 1911, pp. 253, 1212, 1455. 



