THE MICROSCOPE. 



It is necessary, however, before applying this micrometer to 

 the measurement of objects, to ascertain the value of the divi- 

 sions of the scale relatively to the object, since the immediate 

 subject of its measurement is, not the object itself, but the optical 

 image of the object produced in the focus of the eye-piece by the 

 object-glass ; and this preliminary valuation is the more necessary, 

 inasmuch as the relative magnitude of the image, compared with 

 that of the object, will vary with the power of the object-piece. 



To ascertain, then, the value of the divisions of the scale, let 

 another micrometric scale, the divisions of which are known, be 

 placed upon the stage. An image of this scale, magnified as that 

 of an object would be, will then be formed in the focus of the 

 eye-piece, and the other scale will be seen projected upon it. Let 

 the position of the two scales be so adjusted by the stage arrange- 

 ments that the first division of the one shall be projected on the 

 first division of the other. By observing then the next divisions 

 of the two which coincide, the relative value of the scales will be 

 known. Thus if ten divisions of the eye-piece scale exactly cover 

 100 divisions of the other, and if each division of the latter be the 

 1000th of an inch, one division of the eye-piece scale will corre- 

 spond to the 10000th part of an inch in the dimensions of an object. 



It is evident that the value of the divisions of the scale should 

 be determined for each object-piece which the observer uses. 



61. The combination of the camera lucida with the micro- 

 metric scale has supplied a very simple and convenient method of 

 measuring microscopic objects. 



It has been shown in our Tract upon The Camera Lucida, 

 that by that instrument the image of an object magnified in any 

 desired proportion can be thrown upon a sheet of paper upon 

 which its outline can be traced. The micrometric scale is first 

 thus projected, and its divisions, or as many of them as are con- 

 sidered necessary, are traced upon the paper. Another similar 

 series of divisions being traced at right angles to the former, the 

 part of the paper corresponding to the field of view is divided into 

 a system of squares, like those into which a map is divided by the 

 lines of latitude and longitude. The micrometric slide being 

 removed from the stage, the slide with the object is substituted 

 for it, and the observer sees the image of the object similarly 

 magnified projected upon the paper, already spaced out by the 

 squares. He can therefore count the number of squares occupied 

 by its length and breadth, and by the length and breadth of its 

 several parts, or, better still, he can trace its outline upon the 

 paper, so that its dimensions and those of all its parts can be 

 exactly ascertained. Thus, if each division of the scale is the 

 1000th of an inch, the side of each square will represent the 

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