DRAWING TO SCALE FROM MICROSCOPE 



279 



Fig, 150. — Glass disc 

 ruled into squares to 

 serve as tt micrometer 

 eyepiece and micro- 

 area computer. This 

 is to be placed in an 

 ordinary eyepiece. 



or divisions of the scale. This we can determine with a stage 

 micrometer scale, which is simply a glass slip with a scale etched 

 on it divided into tenths and hundredths of a millimeter. Put 

 this in position on the stage and focus with the medium power 

 and find how many of its smallest divisions extend across one 

 of the squares or divisions on the eyepiece 

 scale. Suppose it takes fifteen of them to do 

 this, then we know that any object that is 

 found with the same objective to extend 

 across one of the divisions on the eyepiece 

 scale is exactly .15 mm. in diameter. Try 

 the high power in the same way, and of 

 course the scale on the stage will be magnified 

 more and a less number of its divisions than 

 before will now cover a division in the eye- 

 piece scale. If three of them now Ao this 

 we know that any object under the high 

 power extending across one division of the eyepiece scale has 

 an actual diameter of .03 mm. An arbitrary but accurately 

 determinable scale can now be fixed upon for drawing from 

 the microscope. We may decide to draw 5 mm. long any- 

 thing that covers one eyepiece scale under the medium power, 

 in which case the magnification of the drawing would be .tt= 

 33.3. This will serve to illustrate the method. When the 

 eyepiece micrometer is ruled in the form of squares it can be 

 conveniently used in determining the number of any particular 

 structures in a given area, as in a square millimeter. For in- 

 stance, using the medium power with which an object .15 mm. 

 in diameter would extend across the diameter of the square of 

 the eyepiece scale, suppose we can count 9 stomata in 'the under 

 epidermis of a leaf within one of these squares and we want to 

 determine how many there would be in a square millimeter. 

 That portion of an object which fills one .of the squares would 

 have an area equal to .isX.15 mm. = .0225 sq. mm. How 

 many times would this area have to be taken to make up one 

 square millimeter? Of course the answer would be found by 



