Ch. V] MEASURING WITH THE MICROSCOPE 155 



(§ 239-241). See how many divisions of the eikonometer millimeter 

 scale are required to measure one or more of the divisions of the im- 

 age of the stage micrometer. Suppose it requires 6 intervals or milli- 

 meters of the eikonometer scale to measure the image of 0.03 mm. on 

 the stage micrometer. The size of the object is then 0.03 mm. and of 

 its image 6 mm. The magnification is therefore (§ 228) 6 -t- 0.03 = 200. 



For determining the magnification of a simple microscope the eiko- 

 nometer is placed over the simple microscope as it was over the ocular 

 above. With this instrument, as with the camera lucida, only one 

 eye is used (fig. 81, 100). 



§ 254. Micrometry with the eikonometer. — In the first place the 

 magnification of the microscope must be determined as described 

 in the preceding section; and one must keep in mind the factors which 

 will vary the magnification (§ 235). The object to be measured is 

 put under the microscope and focused and the eikonometer put in 

 position. The virtual image is then measured in millimeters by the 

 scale of the instrument. The size of this virtual image is then divided 

 by the magnification and the result will be the actual size of the 

 object as in § 248. 



For example suppose the long axis of a Necturus' red blood corpuscle 

 measures 9 mm. on the eikonometer scale. If the magnification of 

 the microscope is 200, as found above, then the actual length of the 

 corpuscle is 9 mm. -^ 200= 0.045 mm., or 45 /a. 



§ 255. Micrometry by the aid of the condenser image of a scale. — ■ 

 Probably every one is all too familiar with the cross bars of the window 

 in the field of the microscope. This is, as well known, a real image of 

 the window produced by the condenser at the level of the object. The 

 possibility of projecting a real image at the level of the object is taken 

 advantage of for purposes of micrometry as follows: A lantern slide 

 is made of net hnes (fig. 97) or of any parallel, equidistant lines. The 

 lantern slide is then set up exactly 10 cm. or some other exact distance 

 in front of the microscope. A good hght from the window or from 

 one of the dayhght lanterns (fig. 37-38) must traverse the lantern 

 sUde. This light is reflected up through the condenser by the plane 

 mirror. The condenser will form a real image of the network or par- 

 allel Knes at about the level where the object is placed on the slide.. 



