14 BACTERIA: FORMS AND REPRODUCTION 



Take away the stage micrometer, and in its place put the 

 slide of the bacterium to be measured ; make a drawing of the 

 organism with the camera lucida. Now measure the dimen- 

 sions of the drawing with the scale prepared as above from 

 the micrometer scale, taking care that the eyepiece and objective 

 used in making the paper scale and the drawing of the bacterium 

 to be measured are the same. 



(i) Measure any bacteria which you have mounted and 



stained, 

 (ii) Mount a little yeast in water and measure the size of 



the cells, 

 (iii) Measure the spores of any mould which you can 



obtain. 



(b) A bacterium or other organism may be measured directly 

 by means of an eyepiece scale and a stage micrometer. The 

 eyepiece scale consists of a disk of glass, which fits into the 

 interior of the eyepiece; upon it a fine scale of equi-distant 

 lines is ruled. 



Place the micrometer, ruled in y^ths an d T ^ths of a milli- 

 metre, on the stage of the microscope, and examine it with 

 the eyepiece having the scale 'in it. 



Bring the divisions of the latter into line with those of the 

 stage micrometer, and find the value of one division of the eye- 

 piece scale in y^ths or yj^ths of a millimetre. 



Use the various objectives successively, and record the results 

 for each. 



Place the bacterium or other object to be measured on the 

 microscope stage and determine how many divisions of the 

 eyepiece scale it covers : it is now easy to calculate its real 

 dimensions, as the value of each division of the eyepiece scale 

 is known. 



(i) Measure the objects mentioned above in (i), (ii), (iii) 

 of section (a) in this way, and compare the results 

 obtained by the previous method. 



