CHAPTER III. 



MAGNIFICATION, MICROMETRY AND DRAWING. 



APPARATUS AND MATERIAL FOR THIS CHAPTER. 



Simple and compound microscope (Ch. I) ; Steel scale or rule divided to milli- 

 meters and ^ths ; Block for magnifier and compound microscope ($ 98, 102) ; Divi- 

 ders (§ 98, 99, 102) ; Stage micrometer (g 101) ; Wollaston's camera lucida ($ 102, 

 121) ; Ocular micrometer (£ 112) ; Micrometer ocular ($ 114). Abbe camera lucida 



(U 122-127). 



§ 97. The Magnification, Amplification or Magnifying Power 

 of a microscope or any of its parts is the number obtained by dividing 

 any linear dimension of the image by the corresponding linear dimen- 

 sion of the object magnified. For example, if the image of some ob- 

 ject is 40 mm. long, and the actual length of the object magnified is 

 2 mm. the magnification is 40-4-2=20. 



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

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

 pical or histological drawing is made, the word magnification is fre- 

 quently 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. 



MAGNIFICATION OF A SIMPLE MICROSCOPE. 



§ 98. The Magnification of a Simple Microscope is the ratio 

 between the object magnified (Fig. 4, A B), and the virtual image (Fig. 

 4, A' ; B'). To obtain the size of the image (Fig. 4, A' B'), place the 

 tripod magnifier near the edge of a support of such a height that the 

 distance from the upper surface of the magnifier to the table is 250 

 millimeters. 



As object, place a scale of some kind ruled in millimeters on the sup- 

 port under the magnifier. Put some white paper on the table at the 

 base of the support, and on the side facing the light. 



Close one eye, and hold the head so that the other will be near the 

 upper surface of the lens. Focus if necessary to make the image clear 

 (§ 4). Open the closed eye, and the image of the rule will appear as 

 if on the paper at the base of the support. Hold the head very still, 

 and, with dividers, get the distance between any two lines of the image. 

 This is the so-called method of binocular or double vision in which the 

 microscopic image is seen with one eye and the dividers with the other, 

 the two images appearing to be fused in a single visual field. 



