142 Transactions of the Society. 



V. — New Methods in Microscope Work. 

 By Edward M. Nelson. 



(Read February 1st, 1902.) 



Polarizing with the Microscope. 



Polarizing work with the Microscope is of two distinct kinds : — 



1. The magnification of minute objects under polarized light. 



2. The investigation of phenomena due to the interference of 

 polarized light (known as " rings and brushes "). 



With reference to the first kind the images are much sharper, 

 i.e. more critical, if a pair of tourmalines be substituted for the Nicol 

 prisms usually supplied by the opticians for this purpose. 



One tourmaline should be selected with care ; it should be of a 

 smoky tint with the slightest dash of pink, free from veins and specks, 

 and not less than £ in. in diameter. It should be mounted in a cap 

 to fit over the eye-piece. 



The other tourmaline may be of the ordinary yellow-green variety, 

 but it should be larger than the first ; a rectangle -^ by ^ would 

 be a convenient size and shape. This tourmaline should be mounted 

 in a metal screen, say 2f by 3|-, with an aperture in it of such a 

 size as to exclude all light that does not pass through the tourmaline. 

 This screen may be mounted either on a separate stand, or on an 

 arm attached to the lamp, as the position it is intended to occupy is 

 in front of, and close to the lamp chimney. 



The method of using this apparatus in the first kind of investi- 

 gations needs no explanation. The Microscope is set up in the usual 

 way ; the image of the edge of the lamp flame is sharply focussed by 

 the substage condenser on the object ; the screen holding the tourma- 

 line is placed close to the chimney of the lamp. The object is then 

 examined and, when all the adjustments (collar correction, tube 

 length, size of illuminating cone, &c), are completed, the second 

 tourmaline, in the cap of the eye-piece, is applied, and the eye-piece 

 and object rotated until the desired effect is obtained. If the sub- 

 stage condenser were an achromatic Abbe, and a large axial illuminat- 

 ing cone was required, under the old regime a very large, ond 

 therefore expensive, Nicol would be necessary, because the Nicol 

 would have to be large enough to pass a beam equal to the size of 

 the back lens of the Abbe condenser ; but with this new method a 

 tourmaline large enough to show the middle portion of the lamp 

 flame is sufficient, even when the widest angled cones are employed 



