582 LAI50KAT0KY JiXERClSES 



find in detail a melhod for doing so, and lastly, the practical grower will 

 find formulae and methods for combating the various fungous and insect 

 foes which prey upon his crops and which must be subdued or held in 

 subjection. 



LESSON 1 



Micrometry. — The unit of length used in microscopic measurement is the micron 

 (im) which is the one-thousandth part of a millimeter (o.ooi mm.)- There are 

 four kinds of micrometers in use : the stage, the eyepiece, the step, the filar, or cob- 

 web, micrometer, and where in modem types, the cobweb is replaced by a' finely 

 spun platinum wire. 



Method with Stage Micrometer.- — The stage micrometer is a slide with a scale 

 engraved on it divided to hundredths of a millimeter (o.oi mm.) every tenth line 

 being made longer than the intervening ones, to facilitate counting. 

 I. Attach a camera lucida to the eyepiece of the microscope, 

 z. Adjust the micrometer on the stage of the microscope and accurately focus 

 the divisions. 



3. Project the scale of the stage micrometer on to a piece of paper and with pen, 

 or pencil, sketch in the magnified image, each division of which corresponds to io;t. 

 Mark on the paper the optic combination (ocular objective and tube length) em- 

 ployed to produce this particular magnification. Do this for each of the possible 

 combinations of oculars and objectives, and keep the scales that you have made 

 for future work in measurement, which is accompUshed by projecting the image 

 of the object on the scale corresponding to the optic combination at use in the 

 study. 



Method with Eyepiece Micrometer .—The. eyepiece micrometer is a circle of glass 

 with a scale etched on the surface and suitable for insertion inside of the ocular 

 used during the operation of measurement. The scale is di\-ided to tenths of a 

 millimeter (o.i mm.) or the entire surface of the glass may be etched with squares 

 (o.i mm.), the net micrometer. 



The value of one division of the micrometer scale must be ascertained for each 

 optic combination by the aid of the stage micrometer, thus: 



1. Insert the eyepiece micrometer within the tube of the ocular by placing it 

 on the diaphragm of the ocular, and adjust the stage micrometer by placing it on 

 the stage of the microscope. 



2. Focus the scale of the stage micrometer accurately; the lines of the two 

 micrometers will appear in the same plane. Make the lines on the two micrometers 

 to parallel each other. 



3. Make two of the lines on the ocular micrometer to coincide with those bound- 

 ing one division of the stage micrometer; this is effected by increasing or diminish- 

 ing the tube length; and note the number of included divisions. 



M- Calculate the value of each division of the eyepiece micrometer in terms of 

 by means of the following formula: x = loy. 



Where x = the number of included divisions of the eyepiece micrometer. 

 y = the number of included divisions of the stage micrometer. 



