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APPENDIX A. 



On the Unit of Microscopical Measurement. 



It has for some time been the general practice on the 

 Continent, and is beginning to be so in England, to give the 

 dimensions of microscopic objects in terms of a thousandth of a 

 millimetre, which is called a micro-millimetre, and is variously 

 designated by the abbreviations fi, mk., and mmm. The first 

 abbreviation, being the shortest, is the most generally adopted ; 

 but there seems still to be a prejudice existing against this unit, 

 from a want of knowledge of the advantages which its use 

 confers or of the mode of using it. In the first place, it is 

 always easier to conceive the size of any object, and especially 

 to realise the comparative sizes of two objects, when their 

 dimensions are given in terms of a unit smaller than either ; 

 for instance, it is difficult exactly to comprehend the length 

 represented by ^^ of an inch, and few people can readily 

 compare such dimensions as -^ and ^ of an inch. 



All this difficulty vanishes when the dimensions are expressed 

 as multiples of a small, properly chosen unit, and not as 

 fractions of a large one. For this purpose a fraction of an inch 

 might be adopted instead of a fraction of a millimetre ; but, at 

 any rate in measuring the spores of Fungi, y^u^ of ^ii 

 inch is too large a unit, and loo^ioo of ^i vach. would be 

 inconveniently smaU. It happens that, if we take -^^^ of a 

 millimetre as our unit, we can express the size of the spores of 

 all Fungi, and also of many other microscopic organisms, in 

 the fewest possible figures. For instance, many of the Micro- 



