THE MIOBOSCOPE. 



The arrangement is shown in Fig. 21, where E E and F F 

 are the eye and field glass, the latter having now its plane face 

 towards the object. The rays from the object are here made to 

 converge at A A, immediately in front of the field-glass, and 

 here also is placed a plane glass on which are engraved divisions 

 of a hundredth of an inch or less. 

 The markings of these divisions come 

 into focus therefore at the same time 

 as the image of the object, and both 

 are distinctly seen together. Thus 

 the measure of the magnified image 

 is given by mere inspection, and the 

 value of such measures in reference 

 to the real object may be obtained 

 thus, which, when once obtained, is 

 constant for the same object-glass. 

 Place on the stage of the instrument 

 a divided scale the value of which is 

 known, and viewing this scale as the 

 microscopic object, observe how many 

 of the divisions on the scale attached 

 to the eye-piece correspond with one 

 of those in the magnified image. If, 

 for instance, ten of those in the eye- 

 piece correspond with one of those in 

 the image, and if the divisions are 



known to be equal, then the image is ten times larger than the 

 object, and the dimensions of the object are ten times less 

 than indicated by the micrometer. If the divisions on the 

 micrometer and on the magnified scale were not equal, it be- 

 comes a mere rule-of-three sum, but in general this trouble is 

 taken by the maker of the instrument, who furnishes a table 

 showing the value of each division of the micrometer for every 

 object-glass with which it may be used. 



While on the subject of measuring it may be well to explain 

 the mode of ascertaining the magnifying power of the com- 

 pound microscope, which is generally taken on the assumption 

 before mentioned, that the naked eye sees most distinctly at 

 the distance of ten inches. 



Fig. 21. 



