THE MICROSCOPE BY MEANS OF THEIR OPTICAL CHARACTERS. 615 



ference colours are also affected by the variations in the thick- 

 ness traversed, even by light moving in parallel directions. 



The Bertrand Lens. As the directions-image formed by the 

 objective is small and somewhat inaccessible, it is usual to 

 employ a Bertrand lens, a convex lens, which is placed, when 

 required, in the tube, and forms a secondary directions-image 

 in the focus of the eye-piece. The Bertrand lens is, as a rule, 

 inserted a short distance above the objective, but is sometimes 

 placed higher up, and then occupies only the centre of the tube, 

 so that a large portion of the object-image may, if desired, 

 be left visible. If this be done, the observer can, without losing 

 sight of the directions-image satisfy himself from time to time 

 as to the point on which the microscope is . directed and, if 

 desired, change from one portion of the crystal section to another. 



The Bertrand lens should be capable of being focused by a 

 sliding movement along the axis of the microscope, and it is im- 

 portant that this movement should have sufficient range for the 

 purpose, which is not always the case. 



The Becke Lens. Instead of inserting; the Bertrand lens in 

 the tube, it is possible to obtain the same result more conveniently 

 by placing the Becke lens above the eye-piece. This is a convex 

 lens or system of lenses, similar to a Ramsden eye-piece, which 

 magnifies the directions-image formed in the Ramsden circle of 

 the eye-piece. It should have a focusing movement. 



Isolation of the Directions-Image of a Mineral. If the mineral 

 under examination is not alone in the field, it is desirable to 

 isolate it so that the effects of different minerals may not be 

 blended and thus interfere with one another. 



This object may sometimes be attained by using a closer 

 objective and thus diminishing the extent of the rock-slice or 

 glass slip included in the field. 



A more generally available method, however, is to cut off 

 all light except that reaching the mineral under examination. 

 For this purpose a diaphragm may be placed a little distance 

 below the condenser, which is adjusted so that the image of 

 the aperture in the diaphragm is focused simultaneously with 

 the object. 



In some microscopes the iris diaphragm, attached to the 

 condenser for carrying out the Becke method of determining 

 the relative refractive indices of minerals in thin sections (p. 626). 



