18 HISTOLOGY OF MEDICINAL PLANTS 
If the Bertrand lens is shifted out of the optical axis, one can 
observe the preparation placed upon the microscope stage and, 
depending on its thickness or its double refraction, the inter- 
ference color of the specimen. This interference figure is called 
the orthoscopic image and, accordingly, one speaks of the micro- 
scope as being used as an “‘orthoscope.” 
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 con- 
structed; those of a more elaborate form are used for research 
investigations; others of smaller design for routine investigations. 
