154 



MEASURING WITH THE MICROSCOPE 



[Ch. V 



with the ocular micrometer and dividing it by the magnification of the real 



image, which in this case is 5 diameters. 



Use the fly's wing as object, as in the other cases, 

 and measure the image of the same part. Suppose 

 that it required 30 of the 0.1 mm. divisions = 3 mm. 

 to include the image of the part measured, then evi- 

 dently the actual size of the part measured is 3 mm. -s- 5 

 = § mm., or 0.6 mm., the same result as in the other 

 cases. See also § 253 on the eikonometer. 



In comparing these methods it will be seen that in 

 the first two (1 and 2) the ocular micrometer may be 

 simply ruled with equidistant lines without regard to 

 the absolute size in millimeters or inches of the spaces. 

 In the last method the ocular micrometer must have 

 its spaces some known division of a millimeter or inch. 

 In the first two methods only one standard of measure 

 is required, viz., the stage micrometer; in the last 

 method two standards must be used, viz., a stage mi- 

 crometer and an an ocular micrometer. 



§ 253. Eikonometer for magnification and 

 micrometry. — The eikonometer is something 

 like an eye. It has a converging lens serving 

 in place of the crystalline lens to focus the rays 

 from the eye-piece of the compound microscope, 

 or from the simple microscope upon a microm- 

 eter scale, the scale taking the place of the retina 

 in the eye (fig. 77-78). This scale is ruled in 

 0.1 mm. Above the scale is a Ramsden ocular 

 of 25 mm. equivalent focus, giving a magnifica- 

 tion of 10. The eikonometer scale therefore is 

 a millimeter scale when seen at the distance of 

 250 mm. in the visual field of the normal human 

 eye, and it enables one to put a millimeter scale 

 on the image of any object studied. 



To use it for magnification a stage micrometer 

 is put under the microscope and carefully 

 •focused. Then the eikonometer is put in place 

 over the ocular. The microscopic image of the 

 stage micrometer and the scale of the eikonom- 

 eter will then appear in the same field as with the ordinary ocular 

 micrometer (§ 240). The two sets of lines should be made parallel 



Fig. 96. Wright's 

 Eikonometer. 



(From Sir A. E. 

 Wright's Principles 

 of Microscopy). 



Object. 

 vi Virtual image. 

 Objective. 



ob 



Microscope Ocu- 

 lar, the objective, 

 tube and ocular of 

 the microscope. 



Eikonometer The 

 Ramsden ocular (Ro) 

 magnifying 10 diam- 

 eters, and field lens 

 (//) above the ocular 

 of the microscope. 



es The real image 

 formed at the dia- 

 phragm of the eikon- 

 ometer. 



