64 



In the measurement of most objects the former micrometer is 

 used. This is a scale ruled on glass and placed in a slit in the 

 ocular, or inside, by unscrewing the upper lens of the combination. 



To Determine the Ocular nicrometer Ratio. 



Place the ocular micrometer in position in the slit in the eye- 

 piece, and move the eye lens up or down, until the lines on the 

 glass are distinct. Now place in position the stage micrometer, 

 and the lines on it will appear below under those of the eye-piece 

 micrometer. Move the two scales until the lines of each are parall- 

 el with the other. Measure with the scale of the eye-piece micro- 

 meter, the distance between the lines of the stage micrometer. The 

 ocular micrometer ratio is obtained by dividing the number of 

 spaces on the eye-piece micrometer, required to cover a space on the 

 stage micrometer, by the value of the divisions of the latter. For 

 example, suppose the markings of the stage micrometer were 

 1-100 of a in. in., and the number of spaces of the eye-piece micro- 

 meter required to cover one space in the former was 5, then, the 

 ocular micrometer ratio would be 5-i-l 100 500, i. e., the ratio is 

 500. The value of each division of the ocular micrometer is 1-500 

 with the above conditions in the microscope. The ratio with the 

 several objectives should be ascertained. 



The magnification of an object can be determined, by dividing 

 the size of the image, as measured by the ej^e-piece micrometer, by 

 the ocular micrometer ratio. For example, suppose the dimension 

 of the image of a cell to be 5 divisions of the eye-piece micrometer, 

 and the ocular micrometer ratio is 300, then the size of the cell is 

 5-=-300 = .0166-(-m.m. The size of an object can likewise be deter- 

 mined by measuring the size of the image, under the conditions 

 described for determining the magnification of the instrument with 

 the camera lucida, and then dividing the size of the image by the 

 magnification of the instrument. Suppose the size of the image is 

 5 rn.in. on the paper, and the magnification of the instrument is 300 

 diameters, then the real size of the object is 5^300=. 0166-f-ni.m. 



The unit of micrometry as universally used is the 1-1000 of a 

 m.m. This was suggested by Harting in 1859 and (-tilled, in 1869, 

 by Listing, the micron. It is designated by the Greek letter //. 

 The magnification of an object is then always to be given in mi- 



