THE MICROSCOPE STAND 121 



out of the axis one, or preferably both prisms, so that an 

 immediate change from polarised to ordinary Ught may be made. 



The eyepiece must be provided with cross lines for measuring 

 the angles of crystals by setting first one and then the other 

 edge against one of the lines and measuring the angle of rotation 

 of the stage required to efiect such setting. As the various 

 object glasses and their mountings are never perfectly inter- 

 changeable, a rotating stage must be provided with centring 

 adjustments, so that the axis of rotation can be made to exactly 

 coincide with the optic axis. The Sloan object glass changer 

 described on page 20 is a very useful appliance for adjusting 

 individual object glasses, and is far preferable to a double or 

 triple nosepiece for rapidly changing them. The prisms should 

 be provided with spring clips so that as they are rotated 

 the positions when the prisms are " crossed " may be felt. The 

 lower prism, called the polarising prism, should be large enough 

 to enable the back of the condenser or the object to be fully 

 illuminated, but its size is determined to some extent on the supply 

 of Iceland spar, which cannot always be obtained in large crystals. 

 The upper prism, called the analysing prism, need not be so large, 

 and may be fitted in one of three positions — either immediately 

 over the object glass, in the interior between the two lenses of the 

 eyepiece, or over the top of the eyepiece. If it is immediately 

 over the object glass it makes a slight change in the exact focus 

 of the microscope when pushed in and out unless furnished with 

 a compensating lens or block of glass, which is seldom fitted, as 

 most observers object to the introduction of an extra optical 

 element, when there is no real necessity. With the analyser in 

 this position the use of a quartz wedge is less convenient than in 

 the other two forms. In both the other forms the quartz wedge 

 fits through a slot which is in the focus of the upper lens of the 

 eyepiece. The analysing prism in the interior of the eyepiece 

 is probably the most convenient form, because if fitted above the 

 eyepiece its considerable thickness prevents the eye from being 

 placed in the eyepoint of the microscope (see Fig. 1, page 9) 

 and seriously restricts the field of view. 



The interference rings and brushes of crystals are formed if 

 a wide-angle cone of light be made to pass through the object 

 by a smaU, specially made substage condenser and if this wide- 

 angle cone of light be collected by a wide-angle object glass. A 

 1/6-inch (4-mm.) is generally used for this purpose. The image 

 of these rings and brushes is formed at the back focus of the 

 object glass very close to the back lens of the latter, and there are 

 three methods of observing them. In the microscope in which 

 the analysing prism is immediately over the object glass, the eye- 

 piece may be taken out and the eye placed two or three inches 

 away from the upper end of the tube of the microscope. The 

 image will then be seen, but it will be very small. If the analysing 



