108 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



the eyc-picco r with cross wires ; the analyser is at A. The image is 

 much improved by the use of microscopic objectives (of which the 

 principal focal surfaces are practically plane), instead of simple lenses. 

 The instrument is focused by moving the objective I and tben shifting 

 the eye-piece. The apparatus for concentrating the light consists of a 

 microscopic objective E placed behind a nicol. To use rays of any 

 required refrangibility, a direct-vision spectroscope is employed. The 

 collimator B is moved by a micrometer screw V with divided drum T. 

 The rays, after traversing the prism C and the lens I, form a real spectrum 

 at the principal focus of tbc objective E. The isochromatic curves 

 are then projected upon the spectrum, and a movement of V brings the 

 different colours in succession into the field ; the graduation on the drum 

 will, by previous experiment, give the exact wave-length of the light 



corresponding to any position of 

 the collimator. 



Fig. 8 represents in 1/5 the 

 natural size the apparatus used 

 for the measurement of axial 

 angles ; it is practically that of 

 von Lang. The crystal fragment 

 is held in a spring clip with 

 spherical and rectilinear adjust- 

 ments, and moves under a 

 divided circle reading with 

 verniers to 20". Measurements 

 in oil can as usual be made by 

 the help of the small stage t 

 below the crystal. Tbis appa- 

 ratus may also be used to mea- 

 sure indices of refraction by the 

 method of total reflection ; for 

 this purpose the spectroscope is 

 removed, and the clip is replaced 

 by the two prisms represented 

 half-size in fig. 9, which inclose 

 the section surrounded by a layer 

 of some liquid having a higher 

 refractive index than the section 

 itself. Finally, this part of the 

 apparatus may be used, like the 

 similar Universal Apparatus of 

 Groth, as a Wollaston goniometer. 



Duboscq's Projection Micro- 



- g ■a ^ 1 IS v±:~ scope.* — M. Duboscq's projection 



Microscope (fig. 10) is arranged 

 to carry three objectives, two shown in the fig., the third being at the 

 opposite side of the lantern. This enables different magnifying powers 

 to be used by simply turning the lantern round and without having 

 to screw and unscrew the objectives. Electric ligbt is used for the 

 illumination. 



» Stein, S. T., 'Das Licht im Dienste wiss. Forschung,' v. (1887) pp. 303-5 

 (3 figs.). Also 'Die Optische Piojektions-Kunst im Dienste der exakten Wisseu- 

 schaften,' 1887, pp. 94-6 (3 figs.). 



