Appendix A: Tlic Microscope and Its Optical Principles 153 



correction of aberrations cannot be made, and consequently, a distinct 

 image cannot be obtained. For determination of magnification see 

 micrometer. 



Mechanical Stage. A stage attachment (Fig. 68) for the more accurate 

 manipulation of an object or a series of objects which must be moved 

 about under the objective. The best mechanical stages are provided 



FIG. 68. Attachable Mechanical Stage. 



with scales and verniers so that an object once recorded may be easily 

 found again. They are often very serviceable, especially with high 

 powers. 



Micrometer. A scale for measuring objects under the microscope. 

 The stage micrometer consists of a finely divided scale ( ,V and ik mm.) 

 ruled on glass or metal. It is commonly mounted on a glass slide 

 of standard size. To determine the actual size of an object with the 

 stage micrometer, it is most convenient to use a camera lucida. The 

 outline of the object to be measured is projected on to a sheet of drawing- 

 paper and marked off. The object is then replaced under the micro- 

 scope by the micrometer and the micrometer scale is projected on to the 

 paper. Knowing the actual distance between the lines on the microm- 



