New Methods in Microscope Work. By E. M. Nelso7i. 143 



with any kind of condenser. The images obtained by this new method 

 will be just as critical as those in a Microscope when no polariscope 

 is used. 



In the old method the polarizing prism interfered with the correct 

 performance of the substage condenser; the analysing prism also, 

 when mounted immediately above the objective, destroyed the sharp- 

 ness of the image, and when placed over the eye-piece removed the 

 eye too far from the eye point ; it also caused a deterioration of the 

 image, but not to the same extent as when placed above the objective. 

 Mr. Gordon has shown that the Microscope image is sharpened up 

 when the size of the antipoints is reduced, and that one way of obtain- 

 ing small antipoints is by using large axial cones of illumination. 

 The importance of being able to fill the back lens of the condenser is 

 therefore manifest. 



An apochromatic condenser should not be used in polariscope 

 work, because the fluorite of which it is composed itself polarizes. 



Rings and Brushes. 



1 have previously described * a method of investigating these by 

 the apparatus supplied in an ordinary microscopical outfit, and there- 

 fore will, without repetition, merely point out that large cones of 

 illumination are essential for the demonstration of wide angled' 

 biaxial crystals, and other allied phenomena. The substitution of 

 tourmalines for Nicols is of much advantage, because the illuminating 

 cones may be made as large as possible, and the size of the back lens 

 of the objective on the nose-piece need not be restricted to the width 

 of an analysing prism. The tourmalines are used in the same position 

 as before, viz. one close to the chimney of the lamp, and the other 

 in the cap over the eye-piece. The Zeiss large a * objective is a 

 convenient lens to use at the bottom of the draw-tube. 



It is a pity that these interesting and very beautiful phenomena 

 are not more generally studied. One meets microscopists who own 

 perhaps more than one Microscope, with polariscopes fitted and all 

 the necessary apparatus, and yet who have never seen a ring and 

 brush. 



The Measurement of W.B., W.A. and N.A. 



While on the subject of improved methods of microscopical 

 manipulation, attention might be directed to a most useful piece of 

 apparatus, which hitherto has only been used in connection with a 

 telescope. I allude to Bamsden's Dynamometer, two examples of 

 which are shown in figs. 34 and 35. 



The arrangement of the micrometer screw, as invented in 1 639 

 by William Gascoigne, was very ingenious. On a pinion, which 



* Journ. R.M.S., 1892, p. 683, fig. 81. 



