20 



PRACTICAL HISTOLOGY. 



magnifying powers of the different combinations of the objective 

 and ocular supplied with your microscope : 



17. Measurement of the Size of a Microscopic Object. Sup- 

 pose a red blood-corpuscle is to be measured. Keep both eyes 

 open ; with one look down the tube of the microscope, and the 

 other eye will sec an image of the corpuscles on a sheet of white 

 paper placed beside the microscope. With a pencil mark off the 

 outlines of a corpuscle, or a sketch may be made of one with a 

 camera lucida. Remove the preparation, and for it substitute a 

 stage-micrometer, leaving all the other parts, microscope and paper, 

 as they were. On looking down the tube of the microscope, the 

 lines on the micrometer scale are seen. Make a drawing of these 

 lines. This may be kept for future use, but on the scale should be 

 noted the combination of ocular and objective, and the extent to 

 which the draw-tube has been elongated. The distance between 

 the lines on the micrometer scale being known, say T Jo-th of a 

 millimetre, it is easy to calculate what part of the distance between 

 any two lines corresponds to the size of the corpuscle. 



The most expeditious plan is to use an eye-piece micrometer. 

 A circular flat piece of glass, with a scale ruled on it (fig. 21), is in- 

 serted in the ocular between the field-glass (FG) 

 and the eye-glass (EG) (fig. 22). With this eye- 

 piece focus the scale on a stage-micrometer its 

 markings must be parallel to these in the eye- 

 piece and count the number of divisions of the 

 latter that correspond to one of the former. This 

 must be determined for each combination of lenses 

 w ^h a known length of tube. Suppose the stagc- 

 micrometer to be divided into T ^yths of a milli- 

 metre, and that one of these divisions corresponded 

 to three of those in the ocular, then each of the 

 spaces in the ocular micrometer is equal to - s J-^th of a millimetre, 

 or .0033 millimetre. 



The histological unit of measurement is loooth part of a milli- 



