CHAPTER IV. 



MAGNIFICATION AND MICROMETRY. 



APPARATUS AND MATERIAL FOR THIS CHAPTER. 



Simple and compound microscope ; Steel scale or rule divided to millimeters and 

 ^ths ; Block for magnifier and compound microscope (§ 147, 151) ; Dividers [\ 147, 

 151) ; Stage micrometer (§ 150) ; Wollaston camera lucida (§ 151) ; Ocular screw- 

 micrometer (Fig. 100) ; Micrometer ocular (Figs. 98-99). Abbe camera lucida 

 (Fig. 96). 



§ 145. The Magnification, Amplification or Magnifying Power 

 of a simple or compound microscope is the ratio between the real and 

 the apparent size of the object examined. The apparent size is obtained 

 by measuring the virtual image (Figs. 21, 38). The object for deter- 

 mining magnification must be of known length and is designated a mi- 

 crometer (§ 150). In practice a virtual image is measured by the aid of 

 some form of camera lucida (Figs. 92, 96), or by double vision (§ 147). 

 As the length of the object is known, the magnification is easily deter- 

 mined by dividing the apparent size of the image by the actual size of 

 the object. For example, if the virtual image measures 40 mm. and 

 the object magnified, 2 mm., the amplification must be 40 -=- 2 = 20, 

 that is, the apparent size is 20 fold greater than the real size. 



Magnification is expressed in diameters or times linear, that is, but 

 one dimension is considered. In giving the scale at which a microscop- 

 ical or histological drawing is made, the word magnification is frequent- 

 ly indicated by the sign of multiplication thus : X 450, upon a drawing 

 would mean that the figure or drawing is 450 times as large as the object. 



§ 146. Magnification of Real Images. — In this case the magnifi- 

 cation is the ratio between the size of the real image and the size of the 

 object, and the size of the real image can be measured directly. By 

 recalling the work on the function of an objective (§ 49), it will be 

 remembered that it forms a real image on the ground glass placed on 

 the top of the tube, and that this real image could be looked at with the 

 eye or measured as if it were an actual object. For example, suppose 

 the object were 3 millimeters long and its image on the ground glass 

 measured 15 mm., then the magnification must be, 15 -4- 3 = 5, that is, 



