CH. V] 



MAGNIFICATION BY THE MICROSCOPE 



13* 



make the image fall on at least two of the cones of the fovea, and 

 therefore there would be resolution and any two points would appear 

 as two and not as one. 



MAGNIFICATION 



228. The magnification, am- 

 plification, or magnifying power of 

 a simple or compound microscope 

 is the ratio between the apparent 

 and real size of the object examined. 

 The apparent size is obtained by 

 measuring the virtual image (fig. 77- 

 78). For determining magnification 

 the object must be of known length 

 and is designated a micrometer 

 ( 233). In practice a virtual im- 

 age is measured by the aid of some 

 form of camera lucida (fig. 81, 100), 

 or by double vision (230). As 

 the length of the object is known, 

 the magnification is easily deter- 

 mined by dividing the size of the 

 image by the size of the object. For 

 example, if the virtual image meas- 

 ures 40 mm. and the object magni- 

 fied, 2 mm., the amplification is 

 40 -r- 2 = 20, that is, the apparent 

 size is twentyfold greater than the 

 real size. 



Magnification is expressed in di- 

 ameters or times linear; that is, but 

 one dimension is considered. In 

 giving a scale at which a micro- 

 scopical or histological drawing is 

 made, the word "magnification" is 

 of multiplication: thus, X45O upon 



FIG. 77. SIMPLE MICROSCOPE 

 WITH THE VIRTUAL IMAGE AT 250 

 MM. FROM THE EYE. 



A xis The principal optic axis of 

 the microscope and of the eye. 



/ The principal focus of the mi- 

 croscope. 



A B The object just above the 

 . focus (/). 



B 2 A 2 the retinal image; it is in- 

 verted. 



A 3 B 3 The virtual image at 250 

 mm. from the eye; it is erect. 



Cr Cornea of the eye. 



R Single refracting surface of the 

 schematic eye. 



L The crystalline lens of the eye. 



frequently indicated by the sign 

 a drawing means that the figure 



