102 Appendix A. 



cocci measure about i jt, the spores of Penicillium about 3 /u, the 

 spores of many Myxomycetes about 10 /a, and so on. If we 

 compare these figures with the following : 'ooi mm., '003 mm., 

 or mm., or still more with these : '00004 in., -coo 12 in., 

 0004 in., we see the great saving effected in the trouble of 

 writing down the dimensions, 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 ; I will therefore give an easy method 

 by which the size of the micro-millimetre may be obtained. Place 

 your microscope 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 fine pen subdivide each tenth into five equal 

 parts. Then the value of each of these subdivisions will be 2 ^ 

 and of the whole tenth of an inch, 10 /*. If this scale be care- 

 fully copied on a piece of thin cardboard or other suitable 

 substance, the dimensions of any minute object, drawn by the 

 camera or otherwise on the paper in that position of the 

 instrument, can be easily read off in /t's. With the aid of a 

 deeper eye-piece or higher objective we can magnify the image 

 508 times, and then each small division of the scale will re- 

 present i fji. 



