314 SUMMARY" OF CURRENT RESEARCHES RELATING TO 



MICROSCOPY. 



A. Instruments, Accessories, etc. 



A Projection Spectropolariscope. — F. J.Cheshire {Trans. Optical 

 Soc, li)ll)-20, 21, No. 3, 102-3). A projection spectropolariscope 

 specially adapted for class and lecture work, as shown intiunre on p. 315, 

 consists of a triangular metre-bar upon which t-he necessary fittings are 

 mounted. The fitting A consists of a rotatable disc with a num])er of 

 diiferent sized holes. The fitting B carries a douljle-image prism which 

 can be rotated through 180". The end of the prism casing next to the 

 diaphragm A is fitted with a lens, the focal plane of which coincides with 

 the diaphragm A. A second lens, at the reduced end of the casing, 

 serves to condense the light upon an adjustable slit C. This slit is 

 focussed upon the screen by the projection lens D, the light passing 

 through a double-image prism, or it may be an analyser E and a bottle 

 prism F. In use an arc light is focussed by the usual lantern condenser 

 upon a suitable aperture in the disc A. In this way the light source 

 becomes virtually a sharply defined circular disc. The collimated beam 

 of light passes through the double-image prism in B, one of the result- 

 ing beams passing direct along the axis of the instrument, whilst the 

 second beam is deflected through a sufficient angle to throw it out of 

 the apparatus. In virtue of the sharply defined light source there is 

 practically no intrusion of one beam on the other. The direct beam of 

 plane polarized light (vibrations vertical) is focussed upon the slit C. 

 When therefore a crystal plate, cut parallel to the axis, is placed against 

 the slit with its extinction directions at 45°, and the double-image prism 

 is turned to transmit vertical and horizontal vibrations, two vertically 

 superposed images of the slit are projected upon the screen in the 

 absence of the dispersing prism F, In one of these images the light is 

 polarized in a vertical plane, whilst in the other the light is polarized in 

 a horizontal plane. When the prism F is inserted, these images are 

 drawn out horizontally into spectra, which are seen to be crossed by 

 absorption bands corresponding to the retardation of the plate, one 

 spectrum being complementary to the other. In the case of a plate of 

 axis-cut quartz, the rotation of a Nicol in the fitting E results in a 

 spectrum on the screen with dark absorption bands travelling con- 

 tinuously to the right or to the left according as the particular crystal 

 is right- or left-handed. The apparatus lends itself to the performance 

 of a number of very interesting experiments. The placing of the 

 crystal immediately against the slit results in much sharper definition of 

 the bands than is obtained with the more usual apparatus. (The illus- 

 tration on p. 315 is inserted by the courtesy of the editor of The 

 Transactions of the OjJtical Society.) 



A Spectrometer (made by Messrs. E. R. Watts and Son). — L. C. 

 Martin (Trans. Optical Soc, 1919-20, 21, No. 3, 104-5). This instru- 



