CH. IV] MAGNIFICATION AND MICROMETRY 137 



duces a corresponding change in the ocular micrometer valuation 

 (§ 177. 187, 197)- 



§ 195. 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 micro- 

 scope upon a micrometer scale, the scale taking the place of the 

 retina in the eye (Fig. 16). This scale is ruled in -^ mms. Above 

 the scale is a Ramsden's ocular of 25 mm. equivalent focus, giving 

 a magnification of 10. The eikonometer scale therefore is a milli- 

 meter 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 microm- 

 eter and the scale of the eikonometer will then appear in the same 

 field as with the ordinary ocular micrometer (§ 188). The two sets 

 of lines should be made parallel. See how many divisions of the 

 eikonometer millimeter scale are required to measure one or more 

 of the divisions of the image of the stage micrometer. Suppose it 

 requires 6 intervals or millimeters of the eikonometer scale to meas- 

 ure the image of yf^ mm. on the stage micrometer. The size of the 

 object is then yf , mm. and of its image 6 mm. The magnification 

 is therefore (§ 170) 6 nim.-=- r f T =200. 



For determining the magnification of a simple microscope the 

 eikonometer 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 (§ 176). 



§ 196. 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 (§ 177). 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 eikonometer mm. scale. 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 (§ 184). 



