THE SIZE OF SPORES 159 



Micrometer in a biological research, a brief description of it here 

 will not be out of place. 



The apparatus is provided with a stand, Plate IV., Fig. 26, st, 

 attached to which is a horizontal arm, a, bearing at its end a vertical 

 scale, sc. The scale is divided into fifty parts, with the zero at the 

 top. A carefully prepared plate of glass, 6 mm. thick, p, with 

 parallel upper and lower faces, is attached to a horizontal rod, r, 

 which is fixed to the stand so that it can be rotated about its axis 

 by means of a lever, I. The end of the lever carries a small frame- 

 work in which is placed a piece of glass. On the latter is scratched 

 a fine line parallel to the arm of the lever. The line serves to 

 indicate the position of the lever on the scale. The microscope 

 is provided with a mechanical stage. It also has a slot, si, in 

 the tube above the objective, of such a size that the glass plate can 

 readily be inserted into it. The eyepiece contains a transverse silk 

 thread. 



When the apparatus is about to be used, the glass plate is 

 inserted into the slot so that it becomes entirely enclosed in the 

 microscope tube, which, however, it does not touch. The eyepiece 

 is then rotated until its silk thread comes to be parallel to the 

 rod, r, bearing the glass plate. 



The scale, sc, is calibrated as follows. The lever is first raised 

 until the line in the terminal framework exactly crosses the zero of 

 the scale. A stage micrometer is then placed on the microscope- 

 stage so that its dividing lines are parallel to the thread in the eye- 

 piece. By using the mechanical stage one of the micrometer lines 

 is made to coincide with the thread of the eyepiece. The lever is 

 then depressed. This causes the glass plate to rotate slightly. As 

 one looks down the microscope, the micrometer line appears to move 

 parallel to itself away from the eyepiece line. By depressing the 

 lever far enough, one can make a second stage micrometer line 

 coincide with the eyepiece line. Let us suppose that the distance 

 between the two micrometer lines is 10 /jl, and that the lever has 

 been moved downwards through twenty-six divisions on the vertical 



scale, it is then clear that each scale division has the value — a. 



13 ^ 



By making ten measurements between ten successive divisions of the- 



