MICROMETERS. 
1000th of an inch, and these sides may themselves he easily sub¬ 
divided into ten or 100 parts, so as to carry the measurement to 
lOOOOths or lOOOOOths of an inch. 
In fig. 29 the field of view is represented spaced out in this 
manner, with the outlines of objects traced upon it. 
Such a scale once drawn upon the paper, will serve for the 
measurement of any objects which may he submitted to the 
microscope; but it is most essential that in all such measurements 
the paper be kept at exactly the same distance from the camera, 
and that neither the object-glass, the eye-glass, nor the stage 
shall suffer any change in their relative positions. 
Fig. 29. 
It has been shown that the magnitude of the image received on 
the paper increases with the distance of the paper from the 
camera. If, therefore, the paper be placed at a greater or less 
distance from the camera to receive the image of the object than 
that at which it was placed to receive the image of the micro¬ 
metric scale, the image of the object will be produced upon a scale 
greater or less than that on which the image of the micrometric 
scale was produced, and consequently the one cannot be taken as 
a measure of the other. 
If any change be made in the relative positions of the eye- 
57 
