After the Bertrand lens has been introduced into the optical axis, the interference figure is visible in the back focal plane of the objective. Each point of this interference figure corresponds to a certain direction of the rays of the preparation itself. This arrangement permits observation of the change of the reflection of light taking place in the preparation, this in accordance with the change of the direction of the rays. This interference figure is called the conoscopic image, and, accordingly, the microscope is used as a “conoscope.”
Many types of polarization microscopes have been constructed; those of a more elaborate form are used for research investigations; others of smaller design for routine investigations.
CHAPTER III
MICROSCOPIC MEASUREMENTS
In making critical examinations of powdered drugs, it is frequently necessary to measure the elements under observation, particularly in the case of starches and crystals.
OCULAR MICROMETER
Microscopic measurements are made by the ocular micrometer (Fig. 21). This consists of a circular piece of transparent glass on the centre of which is etched a one- or two-millimeter scale divided into one hundred or two hundred divisions respectively. The value of each line is determined by standardizing with a stage micrometer.
Fig. 21.—Ocular Micrometer Fig. 22.—Stage Micrometer
STAGE MICROMETER
The stage micrometer (Fig. 22) consists of a glass slide upon which is etched a millimeter scale divided into one hundred equal parts or lines: each line has a value of one hundredth of a millimeter.