ZOOLOGY AND BOTANY, MICROSCOPY, ETC, 



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line with the centre of the handle (fig. 150). Hence, the plane of 

 polarization of the light being supposed vertical (i. e. at right angles 

 to the length of a slide placed on the stage) : (1) When the handles of 

 the selenite holders are also vertical, the ray passes through the films 



Fig. 148. 



Fig. 149. 



Fig. 150. 



without change ; (2) When the handles are thrown 45° to the right 

 hand, the maximum retardation due to the sum of the thicknesses of 

 the films is produced ; (3) When anyone of the handles is thrown 45° 

 to the left hand of its central position, the retardation due to that film 

 is subtracted from that of the others. 



Thus it is easy to obtain any given retardation within the range of 

 the series of films without removing the eye from the Microscope. So 

 again, to obtain right-handed circular polarized light, the two thicker 

 films are either thrown out of the field, or placed in the median 

 position, and the handle of the ^ wave film is thrown 45° to the right. 

 To obtain left-handed polarized light it is thrown 45° to the left. 



In my analyser-cap a holder of somewhat similar construction is 

 fitted for a single i wave film, so that it is easy to analyse the light 

 circularly, when desired. If, for special purposes, an entire rotation 

 of a film is desirable, it is only necessary to omit the handle, and mill 

 the edges of the cell on either side of the central groove — in fact, I 

 have several cells thus made. But I prefer the handle from its con- 

 venience in showing the exact optical position of the film. 



I should like also just to call attention to the ease with which almost 

 any microscope-stand can be made to serve as a Norremberg's polari- 

 scope for examining crystals under strongly convergent polarized light, 

 in order to see the rings, &c., round the optic axis. All that is 

 required is, to remove the nose of the main body, and screw into its 

 place the usual system of converging lenses (which answers much 

 better than any microscopic objective), the fourth lens of the system 

 being fitted into the draw-tube. A similar system of lenses is then 

 fitted below the stage, in place of the condenser, and the crystal to be 

 examined is placed on the stage ; or, preferably, supported in a holder 

 like Beck's opaque object-holder, which allows it to be rotated. 



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