124 THE MICROSCOPE 



No. 3223 has the analysing prism in a revolving fitting within the 

 eyepiece. It is a form of the Abbe prism, devised by Mr. E. M. 

 Nelson, which pushes in and out of position. Its great advantage 

 is that it gives the fuU field of view. Its only disadvantage is 

 that in certain circumstances a faint second image of the cross-lines 

 can be observed, but this is of no practical disadvantage. It is 

 provided with spring clicks at positions of crossed Nicols and lines 

 at parallel positions. Below the analysing prism a slot is pro- 

 vided for the insertion of a quartz wedge or mica plate. The top 

 lens of the eyepiece is provided with an adjustment for focussing 

 to either the quartz wedge or the cross- wires, and a Becke lens is 

 provided, fitting over the eyepiece, for examining the rings and 

 brushes of crystals. 



The whole eyepiece pushes into the drawtube with a pin 

 fitting into a slot so that the position of crossed Nicols may be 

 correct when the prisms are set in their clicked position. On 

 either side of the cUcked position a line is marked on the flange 

 of the eyepiece which is 2^° away from the true. position for the 

 total extinction. By setting the analyser to these positions a 

 better determination of the extinction can sometimes be obtained. 

 A shutter with a series of apertures is provided which can be 

 introduced into the field of view to cut ofi aU parts of the field 

 except the centre. A slot is provided at the lower end of the 

 polariser fitting for the insertion of a plate with a fine aperture 

 and a slit, for the testing of refractive index by the Becke shadow 

 test. 

 oircniar A circular mechanical stage (page 52) can be fitted to either of 



the above instruments, and all apparatus of standard microscopes 

 can be supplied, but the substa.ge apparatus is supplied in slightly 

 longer moimts to accommodate for the extra thickness of the 

 mechanical stage. 



meohanical 



sCage. 



