620 RECORD OF CURRENT RESEARCHES RELATING TO 



potuit 'quantum contendere Lynceus.' Duco ad vos Henricum 

 Clifton Sorby." 



Unit of Micrometry."' — Eeferring further to this subject, Professor 

 Eomyn Hitclicock points out that the resolution of the Tiuliauapolis 

 Congress did not deal as is sometimes supposed with a unit for micro- 

 meter-makers to subdivide, for which the -j i^ of a millimetre would 

 probably be the most appropriate subdivision, but with a unit in the 

 sense of the smallest whole number used in giving dimensions. 



The xxfVo iiii^i-5 the so-called micromillimetre, or micra of the 

 French (designated /x), appears to him more suitable for the unit. 

 A few examples of its application may serve to support this view. 



The diameter of a human blood-corpuscle is about 7 ■ 7 /x ( • 0077 

 mm,). In birds, the corpuscles measure from 12 to 14 fx in one 

 direction and from 6 to 8 /x in the other (-012 to -014 mm. x "006 to 

 • 008 mm.). The corpuscles of Proteus anguinus measure 58 fx, and 

 still larger are those of AtnjjMuma tridactjjlum, 175 /x ("058 and 

 •175 mm.). 



If the reader will assume some other unit, as the cm., or mm., or 

 even the ^-J^ ™iii'5 ^^^ endeavour to express the same dimensions in 

 these terms, the advantage of the micra will, he thinks, be obvious. 

 With it a single decimal is sufficient for ordinary purposes. 



Formation of the Paraboloid as an Illuminator for the Micro- 

 scope.! — In an article in the ' American Quarterly Microscopical 

 Journal ' Mr. Wenham gives some useful information relating to 

 practical methods of obtaining parabolic forms. 



The glass parabolic illuminators are ground up to form by means 

 of templates. These may be accurately formed by a purely mechanical 

 method, based on the principle that every section of a cone taken in a 

 plane parallel to the opposite side, is a parabola. Proceed as follows : — 



Fig. 1. 



Turn a cone either of metal or hard wood, between the lathe centres, 

 then on the face plate (which of course should run quite true) chuck 

 the cone on one side, either by cement or clamps as shown in Fig. 1. 



* ' Am. Quart. Micr. Journ.,' i. (1879) p. 235. t Ibid., p. 18G. 



