516 



MICROSCOPY. 



lamps of high intensity, Dr. von Voit found it 

 always desirable, and sometimes indispensable, 

 to prevent interference phenomena by placing 

 under the slide a slip of ground glass or oiled 

 tissue-paper (" J. R. M. 8.," 1884, p. 966). 



It seems advantageous to regulate the strength 

 of the current by means of elevating or de- 

 pressing simultaneously the battery - plates, 



FIG. 30. ELECTRIC ILLUMINATOR; ON MICROSCOPE OF 

 THE AMERICAN TYPE. 



which should be hung and counterpoised, for 

 this purpose, or else sustained by a pillar and 

 ratchet, so as to expose a varying surface to 

 the battery-fluid. 



Various secondary batteries, or accumulat- 

 ors, have been used in connection with the 

 microscopical loop-lamps, with the effect of se- 

 curing greater constancy in the current; but, 

 in the absence of a public service, where they 

 can be charged by steam-power, or by other 

 sources of supply less expensive than the con- 

 sumption of battery elements and fluids, they 

 have as yet failed to prove their superiority 

 to the primary batteries above mentioned. 

 Small dynamo-electric machines for similar use 

 are being rapidly perfected ; Messrs. T. and S. 

 W. Cuttriss, of Leeds, having produced one 

 suitable for a single twenty-candle-power lamp. 

 Such machines are eligible if supplemented 

 with a small water-motor, or some other of 

 the little engines now becoming common for 

 amateur or domestic use. 



Accessories. In the " micro-polariscope " the 

 English style of mounting the analyzer in an 

 adapter, just above the objective, seems to be 

 giving way to the Continental plan of placing it 

 over the ocular ; with decided improvement in 

 respect of definition and facility of manipula- 



tion, which are gained at a islight expense IL 

 narrowness of field. By an adapter fitting 

 over the ocular, and large enough to hold the 

 prism and its mounting, it can be instantly 

 transferred from one location to the other as 

 required. 



The calc-spar prism is still conceded to be 

 superior, both as a polarizer and analyzer, to 

 any of the substitutes that have thus far been 

 offered. In the Hartnack and Prazmowski 

 modification of the Nicol prism, which has 

 been used for years in America though lately 

 said by high authority to be not found in Eng- 

 land, the prism is cut so that the balsam film 

 lies at right angles to the optic axis instead of 

 in the diagonal between the obtuse angles ; the 

 ends are perpendicular to the axis, the length 

 is diminished by about one fourth, and the field 

 is increased from 29 to 35. Prof. S. P. 

 Thompson has still further increased the field 

 by 9 ) y cutting the prism in a principal plane 

 of section, and with the longitudinal axis at 

 right angles to the optic axis of the crystal. 

 These crystals are more costly than the former 

 on account of the greater waste in cutting the 

 spar. Mr. E. Bertrand proposes, in 1884, a 

 plan for still further increasing the field by util- 

 izing the ordinary ray with its greater refrac- 

 tive index. This is accomplished by cutting a 

 prism of flint-glass of index 1*658 through a 

 plane at an angle of 76 43' 8" to the end faces, 

 and between the polished surfaces inserting a 

 cleavage-plate of spar, cemented with a medium 

 of refractive index not less than 1*658. The 

 prism thus obtained is about equal in length to 

 Hartnack's, but has a field of view of 44 46' 

 20". If cut at an angle of 63 26' 15" and ce- 

 mented as before, and then cut and cemented 

 again in a plane symmetrical with the former, 

 a prism of half the length is obtained, with an 

 angle of 98 41' 30". Dr. K. Fenssner's prism 

 likewise consists of a 

 glass prism similar in 

 external form to Hart- 

 nack's Nicol; the 

 usual large and costly 

 prisms of spar being 

 dispensed with, and 

 instead thin cleavage- 

 plates of nitrate of 

 soda being cemented 

 between the cut sur- 

 faces of glass with a 

 mixture of gum-dam- 

 mar and monobromo- 

 naphthalene (" J. R. 

 M. S.," 1883, pp. 428, 

 575 ; 1884, p. 965). 



Prof. Abbe's anal- 

 yzer (Fig. 31) consists 

 of a double-refracting 

 prism of calc-spar, *, 

 achromatized by two glass prisms, g g, which 

 deflect from the field the rays polarized at right 

 angles to the refracting edge, while those polar- 

 ized parallel to the edge pass directly through. 



Aim , ZEB . 



