122 THE MICROSCOPE 



prism is not in this position this method is not permissible, because 

 removing the eyepiece removes the analysing prism, which is an 

 essential to the image being formed. When an eyepiece analysing 

 prism is used, a lens known as a Bertrand lens may be screwed into 

 the lower end of the drawtube or placed into the body through 

 a special slot made for the purpose, and the drawtube pushed up 

 and down until the image is clearly focussed. The Bertrand 

 lens converts the drawtube into a low-power microscope which is 

 focussed to give a sharp image of the back focal plane of the object 

 glass, and a magnified image is obtained. This method sufiers 

 from the inconvenience that the drawtube must generally be 

 removed to put in the Bertrand lens, and that it is troublesome 

 to use a sliding drawtube in a petrological microscope, as it may 

 interfere with the accuracy of the crossed position of the prisms. 

 The best method of observing the rings and brushes is by means 

 of a small microscope called a Becke lens, which fits on to the 

 top of the eyepiece and gives a highly magnified image of the 

 eyepoint or Ramsden circle of the microscope (Fig. 1, page 9). 

 The image of the rings and brushes is, as previously mentioned, 

 in the back focal plane of the object glass, but this is reproduced 

 by the eyepiece in the eyepoint, and it may be examined equally 

 well in this position. The use of the Becke lens does not interfere 

 with any of the adjustments of the instrument, and is to be 

 preferred to any other plan. 



Petrological microscopes cannot be thoroughly explained 

 without considerable discussion of the theory of polarised Ught, 

 which is not attempted in this book. There are excellent books 

 on Petrology to which the student is referred, and to whom 

 the following technical description of a petrological microscope 

 will then appeal. 

 standard The Standard London Petrological Microscope is made in two 



SSSpe! ^°^"^ (^°^- 3222 ^^^ 3223). Both forms have the rack and 

 pinion spiral coarse adjustment and the double-speed fine adjust- 

 ment of the Standard London Microscopes ; both have a circular 

 rotating stage divided in degrees and cross-finder divisions on 

 the sxirface with centring screws to set the axis of rotation in the 

 optic axis ; they both have cross- wires to the eyepieces, a polariser 

 in a swing-out fitting below the stage, and a wide-angle series 

 of converging lenses in a sliding fitting in the stage. This 

 condenser can also be fitted in an independent swing-out and 

 focussing arm, which enables the condenser to be thrown out of 

 the optic axis in a manner similar to the polariser. The polariser 

 is provided with spring clicks at positions of crossed prisms and 

 lines at parallel positions. No. 3222 has an analyser in a push- 

 out fitting above the object glass at the lower end of the body. 

 Below this is a slot covered by a revolving tube when not in use, 

 for the insertion of mica or quartz plates. A Becke lens slides 

 over the eyepiece for examining the rings and brushes of crystals. 



