MICROSCOPY. 



517 



The system is mounted inside a Huygenian 

 ocular between the diaphragm, d, and the eye- 

 lens, e. Mr. C. D. Ahrens's polarizer, pro- 

 posed in 1884, consists of three wedges of spar 

 (, p, s, Fig. 32), the optic axis in 

 the two outer wedges, s , being 

 parallel to the refracting edge, 

 while in the middle wedge, jt>, it 

 is perpendicular and in a plane 

 bisecting the refracting angles. 

 A prism, g, of glass is added, which 

 achromatizes one ray and corrects 

 its deviation, but increases the de- 

 viation of the other so much that 

 it does not interfere with ordinary 

 observations. The prisms are all 

 cemented together with balsam, 

 forming a compound prism whose 

 length is scarcely more than twice its breadth, 

 and which can be used over oculars A and B 

 without cutting down the field of view. 



For goniometry the modern graduated round 

 stages, in connection with the lines of an ocu- 

 lar micrometer, are found a sufficient as well 

 as economical arrangement, except for the work 

 of specialists, who require the lithological mi- 

 croscopes above mentioned. Even to an abso- 

 lutely simple round stage, a graduated revolv- 

 ing object-carrier, useful also for other purposes, 

 can easily be added. When made separate 

 from the stand, the goniometer is conveniently 

 combined with the polariscope analyzer; or, 

 as in an excellent recent construction by Mr. 

 Zentmayer, with the cobweb micrometer. 



As a micro-spectroscope the Sor by-Brown- 

 ing ocular arrangement, notwithstanding its 

 too limited dispersion, continues to be the 

 standard form on account of its general appli- 

 cability and its ready comparison of different 



FIG. 33. SPECTRAL OCULAR. 



spectra. Its rather unsatisfactory facilities for 

 mapping or measuring the spectra have been 

 improved by the addition of various illuminated 

 micrometers whose image is projected from the 



side into the upper part of the tube and reflect- 

 ed from the surface of the upper prism into the 

 field of view. Fig. 33 shows at one half size 

 the form manufactured by Zeiss, having below 

 an ocular with an adjustable slit A serving as 

 its diaphragm, above in the tube J a set of di- 

 rect-vision dispersing prisms between the eye- 

 lens and the eye, and at N the micrometer scale 

 whose image, formed by the lens R and pro- 

 jected into the spectrum by reflection from the 

 upper surface of the top prism, enables the po- 

 sition of dark or bright lines in the spectrum 

 to be determined by a direct reading of their 

 wave-lengths in decimals of p.. The scale 

 reads to the second place of decimals, which 

 may be further subdivided by the eye; and 

 lithographed sheets with scales enlarged to 100 

 mm. are used for recording observations. At 

 A there is also a supplementary stage and prism, 

 not shown in the cut, for the comparison of 

 spectra. For greater dispersion the prisms have 

 been sometimes transferred, at Mr. W . Crookes's 

 suggestion, to an adapter at the bottom of the 

 draw-tube or of the nose-piece ; the slit being 

 placed in the sub-stage, and its image being pro- 

 jected upon the object by means of an object- 

 ive used as achromatic condenser. This ar- 

 rangement is applicable to the binocular, and 

 it is easily combined with a double-image prism 

 near the slit, or with an ordinary pair of JSicol's 

 prisms as polarizer and analyzer, in all which 

 cases remarkable results may 

 be attained. The slit may be 

 mounted on a centering nose- 

 piece attached to the bottom 

 of the sub-stage, or on the 

 sliding plate of the " iris illu- 

 minator" (Fig. 25), in which 

 latter case it replaces the iris 

 diaphragm, and can be cen- 

 tered by the decentering ad- 

 justment of the plate. From 

 his experience with such an 

 arrangement, the writer is 

 inclined to believe that the 

 prisms should be located 

 somewhere at the objective 

 and not at the ocular end of 

 the instrument. A late form 

 of mounting by Mr. Sorby 

 (Fig. 34) places the prisms C 

 between a specially arranged object-glass A 

 and a cylindrical lens F for lengthening the 

 spectrum, the slit being at K, a quartz standard 

 scale for measuring the position of the bands at 

 H, the lines of whose interference spectrum are 

 reflected into the field of view by the small 

 right-angled prism G, and a collecting lens I, 

 which when in use is situated immediately 

 above the object. 



The micro - spectral apparatus devised by 

 Prof. T. W. Engelmann for the study of assimi- 

 lative action under the influence of luminous 

 rays of different wave-lengths replaces, when 

 in use, the ordinary illuminating apparatus, 

 and consists of a plane mirror, an adjustable 



FIG. 34. SPECTRO- 

 SCOPE FOR BINOC- 

 ULAR. 



