MICROMETRY MICROMETRIC MICROSCOPES 



189 



to a cross-bar sliding upon a strip of wood 25 cm. long graduated 

 in centimeters; this strip is attached to two small blocks, each 

 the thickness of the base of the microscope stand. One block 

 is notched at the end so as 

 to permit its being always 

 placed exactly in the same 

 positive (Ives' method). 

 The best results are ob- 

 tained when a strong source 

 of artificial light is employed 

 to illuminate the screen and 

 a piece of ground glass is 

 placed between the radiant 

 and the scale-screen. 



The values of the scale 

 are determined for three or 



FIG. 122. Full Size Scale for Micrometry 

 by Projection of Image by the Substage 

 Condenser. 



more positions of the gradu- 

 ated strip and the results plotted upon coordinate paper. 

 This is accomplished as folloi*$ : Place a stage micrometer upon 

 the stage of the microscope,^center and focus sharply using say 

 a 1 6 mm. objective and 7.5 X eyepiece. Raise the substage con- 

 denser until the upper lens almost touches the object slide; open 

 the iris diaphragm. Tip the plane mirror to one side and at the 

 proper angle to throw an image of the scale into the condenser. 

 Lower the condenser while looking into the microscope until 

 the scale becomes clear and sharp. Turn the stage micrometer 

 so that its graduations become parallel with those of the real 

 image of the scale-screen. Move the stage micrometer until any 

 line of the stage micrometer coincides with a line on the scale 

 image. Count the number of division of the scale included in 

 a division of the stage micrometer. Calculate the value for 

 one division of the scale. Record the distance of the scale from 

 the mirror as shown on the graduated strip and compute the 

 value in microns as obtained for this position. Move the scale 

 carrier to a new position and determine the value of a scale divi- 

 sion as described above. In like manner find the true value for 

 a third position. Plot the results upon a fairly large sheet of 



