32 



VIEWING AND ILLUMINATING 



engaged eye, so that it may be seen by that eye, overlapping or 

 lying by the side of the magnified picture of tbe other scale. Then 

 move the rule till one or more of its known divisions correspond 

 with a number of those in the magnified scale, and a comparison of the 

 two gives the magnifying power. 



20. MICROMETER EYE -PIECE. The mi- 

 crometer eye-piece is in many instances a very 

 useful apparatus, and is the invention of a 

 Mr. Ramsden. When it is stated, that we 

 sometimes require to measure portions of animal 

 or vegetable matter a hundred times smaller 

 than any divisions that can be artificially made 

 on any measuring instrument, the advantage of 

 applying the scale to the magnified object will 

 be obvious, as compared with the application 

 of engraved or mechanical micrometers to the 

 stage of the instrument. 



The arrangement is shown in the accom- 

 panying figure, where E E and F F are the 

 eye and field-glasses, 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 1 -100th of an inch, or 

 less. The markings of these divisions come Fig. 17. 



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 instru- 

 ment 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 corres- 

 pond with one of those in the image, and if the divisions are known to 



