172 AVOIDANCE OF INVERSION; ERECT IMAGES [Ch. VI 



drawing 4 cm. or 40 mm., the scale of the drawing or its magnification 

 is 40 -r- 0.1 = 400. 



§ 276a. For diagrams and other large objects a very serviceable micrometer 

 can be made by using the 10 cm. metric rule (fig. 104) as object and making a 

 negative of it on a lantern slide exactly natural size or half natural size. 



10 CENTIMETER RULE. 

 The upper edge is in millimeters, the lower in centimeters. 



THE METRIC SYSTEM. 



UNITS. The most commonly used divisions and multiiles, 



I Centimeter (cm), o.oi Meter; Millimeter (mm.), o.ooi Meter : Micron ([L>, o.ooi 

 the meter for ■< Millimeter; the Micron is the unit in Micrometry, 



length ( Kilometer, iooo Meters; used in measuring roads and other long distances. 



the gram for I Milligram (mg.), o.ooi Gram. 



weight I Kilogram, iooo Grams, used for ordinary masses, like groceries, etc. 



the liter for j Cubic Centimeter, (cc), o.ooi Liter. This is more common than the correct 

 capacity. I form, Milliliter. 



Divisions of the Units are indicated by the Latin prefixes; deci. o.i ; centi, o.oi ; milli, o.ooi ; 

 micro, one millionth (o.oooooi) of any unit. 



Multiples are designated by the Greek prefixes; dcka, 10 times; hecto, ioo times; kilo, iooo 

 times; jnyria, 10,000 times; mega, one million (1,000,000) times any unit. 



Fig. 104. Metric Scale and Summary of the Metric System. 



Avoidance op Inversion 



§ 277. It is desirable to make drawings like the object without 

 any inversion whatsoever, provided the object has rights and lefts, 

 etc. For structural detail like cells, etc., it makes no difference 

 whether the image is erect or not, but with symmetrical organs and 

 animals it is very confusing to have the parts inverted in the drawing. 

 For example, it is unsatisfactory to have the liver shown as if on the 

 left side and the heart on the right side. 



In order to avoid inversions, it is necessary to know what inver- 

 sions are produced by the different optical appliances used to assist 

 in drawing. Then one can so arrange the object that the image 



