XI. WHEN TO USE SPECIAL MICROSCOPES 375 



and positive oculars are practically distortion-free, but often have 

 too much curvature of field for photomicrograph3^ Special oculars 

 are available and should be used for projection, photography, and 

 special applications. A comparison ocular with a split field may be 

 placed over two microscopes when close comparisons are required. 

 The demonstration ocular makes simultaneous observation possible 

 for two observers. 



Microspectroscopes have been designed for use with the micro- 

 scope (3,4,52) and microcolorimetry and refractometry are also pos- 

 sible {3,4,53). 



Once a detail is located on a slide, it may be important to mark its 

 place so that it may be found another time. This can be done crudely 

 with a drop of ink just below the object, but better by recording the 

 coordinates of a graduated mechanical stage, or by replacing the ob- 

 ject slide with a Malt wood Finder or other slide having numbered 

 squares on it and then recording its position number; reversing the 

 process leads one back to the same area on the specimen. Other 

 useful methods mark the cover glass over the specimen {54)- 



3. Counting and Measuring 



A ruled scale placed in the eyepiece with the ruled side next to the 

 top of the diaphragm is seen superimposed on the image of the speci- 

 men and may be used for counting or measuring. The value of the 

 scale unit depends on the magnification of the ocular and objective 

 and is obtained by calibration with a stage micrometer. Any change 

 of the magnification resulting from substituted optics, readjustment 

 of the draw tube of the microscope, or of the interpupillary distance 

 when inclined ocular tubes are present will change the unit value and 

 require recalibration. With an adjustable draw tube the distance 

 subtended by the reticule scale can be changed, within limits, to give 

 convenient integral values. Since the spherical aberration is changed 

 with the change in tube length, this method of adjustment should not 

 be used for critical purposes. 



Filar, or screw eyepieces may be obtained with movable scales 

 for precision measuring. Measurement can be made also with a 

 mechanical stage with graduated scales. A special integrating me- 

 chanical stage with several movements may be used for estimating 

 percentage composition of a specimen by means of area and/or vol- 

 ume determinations. 



