642 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



The Observation Oculars used have an enlarged field of view about 

 double the extent of ordinary oculars. In their image plane is a disc 

 with a round and a square diaphragm, so that a round or a square 

 periphery can, as desired, be given to the image. The latter serves for 

 the quicker enumeration of constituent parts in any section. The other 

 arrangements are practically identical with those of ordinary large 

 polarisation-microscopes. The object stage T and the Nicol circle T x 

 are graduated in degrees and their verniers read to 5 minutes. The 

 Bertrand lens B and the analysing Nicol N can be cut out of the pencil 

 of rays ; the latter by means of the spring rod/; the former by means 

 of a small clip swung back during rotation. The tooth-wheel gear has 

 the well-known arrangement for avoiding dead-way in the teeth.* In 

 lieu of the cross-slit stage this instrument has for swift investigation of 

 a section a simple slide arrangement, by means of which a slide can be 

 pushed by hand-motion freely in two rectangular directions. For upper- 

 surface illumination an adjustable holder for an electric glow-lamp G 

 can be installed near the object stage. 



The Planimeter- Ocular is seen at P in the right of fig. 144. It is 



Fig. 145. 



used for determining the volume-proportion of any mineral constituents 

 in a thin rock-section. In the image-plane of the ocular two micro- 

 meter scales (fig. 145) perpendicular to each other are cut on glass. They 

 serve to give ordinates and abscissas ; the scale for the former being 

 fixed ; the latter adjustable by rack-and-pinion. Their combined 

 motions explore a space of one square cm. The planimeter-ocular is 

 applied in such a way that corresponding to the grain of the rock the 

 abscissas are adjusted on a particular graduation of the ordinates and the 

 condensation index is read off for the various parts. The section is 

 then rotated, or the planimeter-ocular rotated, through 90°, and the 

 reading repeated in the perpendicular direction. 



Microphotoscope, or Military Staff Map Loup.f — This arrangement 

 is designed by its inventor, 0. Vollbehr, for the avoidance of the in- 



• Zeit. f. lnstrumentenk., svi. (189G) p. 17. 



t Extract from KriegBtech. Zeitschr.. 1905, Heft. 1", 12 pp. and 3 figs.. Berlin, 

 E. S. Mittler and Sohn : Zeitsebr. f. lnstrumentenk., xxv. (April 1905) pp. 117-18 ; 

 Central-Zeit. f. Opt. u. Mech., xxvi. (May 1905) p. 106. 



