96 A MANUAL OF BACTERIOLOGY 



one. For this purpose a fraction of an inch might 

 be adopted instead of a fraction of a millimetre 

 (mm.) ; but, at any rate, in measuring the spores of 

 fungi, TWOTT f an i ncn is too large a unit, and 

 TWO-OIF of an inch would be inconveniently small. 

 It happens that, if we take TFOTJ- of a millimetre as 

 our unit, we can express the size of the spores of all 

 fungi, etc., in the fewest possible figures. For in- 

 stance, many micrococci measure about 1 //,, the 

 spores of Penicillium about 3 //,, the spores of many 

 Myxomycetes about 1 //,, and so on. If we compare 

 these figures with the following: O'OOl mm., 0'003 

 mm., O'Ol mm.; or, still more, with these: 0*00004 

 in., 0*00012 in., 0'0004 in. we see the great saving 

 effected in the trouble of writing down the dimen- 

 sions, quite apart from the greater readiness with 

 which they can be compared with one another. 

 But perhaps the difficulty with some is that of 

 realising and actually applying this unit; we will 

 therefore give an easy method by which the size of the 

 micromillimetre may be obtained. Place the micro- 

 scope in such a position that the image projected 

 upon a piece of white paper is magnified 254 times : 

 this can easily be done by a quarter-inch objective, 

 with the use of the draw-tube, or by placing the 

 paper at a greater distance than ten inches from the 

 eye-piece. Let this position be marked, so that the 

 microscope can be placed in it again at any time. 

 Now copy on the paper, from a scale, an inch 

 divided into ten parts, and with a line pen subdivide 

 each tenth into five equal parts. Then the value of 

 each of these subdivisions will be 2 /A, and of the 



