impossible in the higher powers, as in these, focal 

 adjustment for different depths is required. 



Furthermore the accommodation of the eye is also 

 a factor as it varies with different persons and thus, to 

 a certain extent, is a matter of individuality. 



Flatness of Field. The field of view or in 

 brief the field in a microscope is the area which is 

 observed by looking into the eyepiece. While its area 

 is constant in an eyepiece, the size of the area being 

 determined by the diameter of the diaphragm, it is 

 variable with the change of objectives, the field or 

 visible portion of the object becoming smaller with the 

 increase in power. Angular aperture has no influence 

 on the size of field. The field is said to be flat when 

 all the parts of the image within its area are sharp at 

 the same time. 



When not fiat, it will be found that as the edge 

 of the field is approached the image becomes more and 

 more indistinct, and that the objective must be corre- 

 spondingly adjusted ; in many cases it remains indis- 

 tinct or blurred, and this may be considered a most 

 serious fault. Lack of flatness may be due to the 

 un evenness of the object, and in order to test the 

 objective for flatness, an object of assured flatness 

 should be used, such as a stage micrometer which con- 

 sists of a series of very fine parallel lines cut into 

 glass and very accurately spaced according to definite 



95 



