i6o 



RESEARCHES ON FUNGI 



stage micrometer, one can obtain a very accurate value for each 

 division of the plate micrometer scale. 



When the scale has been calibrated by the method just described, 

 one proceeds to measure the size of spores. These are mounted in 

 water on a glass slide and covered with a cover-glass in the usual 

 way. The lever is placed at zero. One then finds a spore with the 

 axis to be measured directed at right angles to the eyepiece line. 1 

 Let us suppose that one wishes to measure the long axis. With the 

 mechanical stage one moves the spore so that one end of it just 

 touches the eyepiece line (Fig. 54). One then depresses the lever 



from the zero of the scale, and as one 

 does so the spore appears to move 

 across the eyepiece line from left to 

 right, until finally it comes to touch it 

 with its other end. At this point one 

 ceases to depress the lever and reads 

 off the number of divisions on the scale 

 through which it has been moved. By 

 measuring twenty-five spores in this 



Fig. 54.— Measurement of a spore Qne can obtain a very good aver- 



by means of the Poynting Plate J ' J ° 



Micrometer. By depressing the a ge on the scale for the dimensions 



lever the spore appears to move . . „ 



across the eye-piece line ee from required. Since the actual value ot a 



position A to position B. ^ micrometer scale division has 



previously been found by calibration, the dimensions of the spores 

 can readily be calculated. 



The advantages of the apparatus are : (1) Its optical soundness 

 — each division on the vertical scale has the same value ; (2) the 

 apparatus is entirely detached from the microscope, so that, when 

 the lever is moved, the microscope cannot be shaken in any way ; 

 (3) the accuracy with which the scale can be calibrated ; (4) its 

 simplicity; (.5) the speed and ease with which large numbers of 

 observations can be made with it. 



The range of variation in the sizes of spores may be gathered 



1 With the addition of a rotating stage one could place any spore with its axis 

 in the desired dh'ection. My microscope, unfortunately, was without this refine- 

 ment. There is, however, very little difficulty in finding as many spores as one 

 requires with their axes in the right direction. 



