ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



C85 



Fig. 74. 



MICROSCOPY. 



a. Instruzaents, Accessories, &c.* 

 (1) Stands. 



Binocular Microscopes (Abrens, Goltzsch, and Holmes). — It is no 

 doubt somewliat rash to attempt to assert that we have reached a 

 final point with regard to any branch of microscoi:)y, but at the 

 same time we are inclined to think that when this notice has been 

 published, all the forms of Binocular Microscope will have been de- 

 scribed that it can be at all worth any one's while to invent — at any rate, 

 for any purpose of practical use. The only forms which have hitherto 

 stood the test of use, are those of Nachet, Wenham, and Stephenson. 



Ahrens's Polarizing Binocular Microscope.- — Hitherto there has been 

 some difficulty, Mr, C. D. Ahrens considers, in using binocular Micro- 

 scopes with polarizing apparatus, mainly on account of the practical 

 difficulties attending the use of analysing prisms wi'.h the double tube 

 for the two eyes. "My invention has for its object the construction of 

 binocular Microscopes in which the difficulty of analysing the light is 

 obviated by en ^^loyiug actual polarizing surfaces 

 to divide the ravs as they emerge from the object- 

 glass. This I prefer to accomplish in the follow- 

 ing manner: — Over the object-glass is set a prism 

 of black glass having "' horizontal side upwards as 

 close to the object-glass as is convenient, and having 

 its two faces symmetrically inclined to the axis of 

 the object-glass at angles of about 57°, which is 

 approximately the angle of complete polarization. 

 The bundle of rays is thus reflected at the proper 

 angle, and divided at the same time into two parts. 

 These parts or rays, passing obliquely right and 

 left, are then reflected up the two tubes to the two 

 eye-pieces, either by two total reflection prisms or 

 by polished metal surfaces. Light polarized by a 

 suitable polarizer before traversing the object, will 

 be analysed by the said prism or prisma placed 

 above the object-glass." The prisms are shown in 

 fig. 74. ^ 



Goltzsclis second Binocular Microscope^ — We 

 have already described J Herr H. Goltzsch's first binocular Microscope, 

 one of the features of which was the use of small telescopes for eye- 

 pieces. He subsequently announced an improvement, by which the 

 inconvenient vertical stage is dispensed with. 



Close above the objective a rectangular prism is placed, the antex-ior 

 acute angle of which is in the axis while the hypothenuse surface is 

 inclined at an angle of 3^^, so that half the pencil from the objective is 

 diverted, to the extent of 7' from the axis, by total reflection. Behind the 



* This subdivision contains (1) Stands; (2) Eye-pieces and Objectives; (.3) Illu- 

 minating and other Apparatus ; (4) Photomicrography ; (5) Microscopical Optics 

 and Manipulation ; (6) Miscellaneous. 



t Carl's Eep. f. Exper.-Physik, sviii. (1882) pp. 27-32 (1 fig.). 



♦ See this Journal, 1882, p. 9.i. 



