156 ELEMENTARY CHEMICAL MICROSCOPY 



and employed by Sorby in refractive index determination in 

 1878. Again revived by A. E. Wright in 1890. Thoroughly 

 tested out by Ives in 1903 and independently rediscovered by 

 Clendinnen in I9I0. 1 And yet in spite of the many times this 

 principle of employing a scale of variable value as a standard 

 has been independently discovered and its desirable features 

 pointed out, it is almost never referred to in manuals devoted to 

 microscopy. 



By means of the mirror and the Abbe condenser, it is possible 

 to project into the plane of the object lying upon the stage, the 

 image of a scale whose value has been ascertained. Both scale 

 and object are magnified together and it therefore follows that 

 no matter what may be the combination of objective and ocular 

 employed, the value of the divisions of the scale image will 

 remain unchanged, provided that the distance of the scale from 

 the condenser is not altered. Any change in the distance of 

 scale from mirror and condenser will be accompanied by a pro- 

 portional change in the size of the divisions of the scale in the 

 image projected into the plane of the object. 



In micrometry, by means of ocular micrometers, we are re- 

 stricted to the single ocular, containing the scale, and to a fixed 

 draw-tube length. To obtain a different magnification, one is 

 obliged to change objectives. This means that a new ocular 

 micrometer value must be employed and a record kept for every 

 change in objective. Moreover, the actual sizes of the divisions 

 seen in the eyepiece micrometer are constant and cannot be 

 changed. 



In micrometry, by means of a scale image projected by the 

 condenser, we have merely to record the distance of the scale 

 from the microscope in determining its value and we may then 

 adopt any possible combination of objectives, oculars or tube 

 lengths, without change of value. 



The scale employed may conveniently consist of a glass posi- 

 tive obtained by photographing any suitable scale drawn in 

 ink or pencil, either as a linear scale or ruled in squares, 1 and 



1 See Ives, Journ. Frank. Inst., 154, 73; Clendinnen, J. Roy. Micro. Soc., 

 1910, 368. 



