MICROMETRY AND THE CAMERA LUCIDA. S3 



difficult to measure small objects, since, as mounted on 

 the slide, they tend to lie obliquely across the scale, and 

 their edges would only rarely coincide with its lines. 

 As a rule, therefore, some indirect method of measure- 

 ment is adopted. 



2. The Ocular Micrometer. — ^The most satisfactory 

 method of measurement with the microscope involves 

 the use of another scale, called the ocular micrometer. 

 This is a circular piece of glass cut to fit in the eyepiece 

 of the microscope and bearing a scale made up of lines 

 at an equal, but not necessarily a known, distance apart. 

 The micrometer is to be placed inside the eyepiece, 

 resting on the sliding diaphragm inside it, which should 

 be so adjusted as to lie in the plane of the real image 

 formed by the objective. Thus the image will lie on 

 the scale, and may be measured by it. The value ob- 

 served is therefore the size of the virtual image of an 

 object formed by the objective in terms of an arbitrary 

 scale. In order to obtain actual dimensions the ocular 

 micrometer must be standardized in order to determine 

 the value of its divisions as compared to the image of an 

 object of known size. For this purpose it is simply nec- 

 essary to place a stage micrometer under the microscope 

 and focus upon it with the ocular micrometer in position. 

 The ratio of ocular divisions to stage divisions (or rather 

 to the images of stage divisions) can then be read off 

 and, knowing the actual value of the stage divisions, it is 

 easy to calculate the number of microns on the stage 

 which correspond to one ocular division. When once 

 the ocular micrometer is standardized for a given system 



