

MICROSCOPY. 



515 



have large, flat, horizontal reservoirs, with a 

 low built burner attached near one edge. 



Lately, the electric light has been employed 

 in microscopic illumination, with results as yet 

 imperfect, but not unimportant in themselves, 

 nor doubtful in their promise of a more ade- 

 quate success. The incandescent or loop lights 

 of small size are mostly employed, giving an 

 illumination remarkably steady and brilliant, 

 exceptionally free from heat-rays, of high re- 

 solving capabilities on account of its small 

 proportion of yellow rays, white enough to 

 show colors as they appear in daylight, and 

 well adapted to spectroscopic work, photo- 

 micrography, and resolution with high powers. 

 They were brought into this service by Dr, 

 Van Heurck, of Antwerp, whose remarkable 

 photographs of A. pellucida were made by sub- 

 stituting a Swan lamp of six volts for the wick 

 of a Nelson-Mayall microscope-lamp. But as 

 large batteries are inconvenient as well as ex- 

 pensive, and electricity supplied from public 

 stations is not yet generally available, smaller 

 lamps with diminished resistance have been 

 substituted, placing them enough nearer to the 

 object to secure sufficient intensity of light. 

 Mr. C. H. Steam, of 

 England, adopted a 

 Swan lamp whose fila- 

 ment is ^ inch long 

 and y!^ inch in diam- 

 eter, and capable of be- 

 ing rendered fully in- 

 candescent by two to 

 three Grenet or four 

 to six Leclanch6 cells; 

 a light of 2 candle- 

 power being obtained 

 with an electro-motive 

 force of 3| volts. For 

 the purpose of regulat- 

 ing the intensity of the 

 _ light, any required n urn - 



**dL i* 5 *^^-. ber of cells is switched 

 into the circuit, or an 

 adjustable resistance- 

 coil is introduced. The 

 lamps are mounted upon 

 a stand, as in Fig. 28, or 

 upon a bull's-eye stand, or are attached to the 

 microscope itself above the stage for opaque, 

 and close below the object-slide for transparent 

 illumination, and below the sub-stage for use 

 in connection with the achromatic condenser, 

 polariscope, or both (" J. K. M. S.," 1883, p. 29). 

 An image of the source of light formed at 

 the plane of the object on the stage, which is 

 generally preferred with lamp or white-cloud 

 light, is not equally desirable with the loop, 

 which is therefore placed at such distance that 

 the conjugate focus will be slightly beyond 

 the stage, and the light diffused to the extent 

 found desirable, or with low powers opal in- 

 stead of clear glass is used for the lamp. A 

 suitable spread of the light over the object, 

 and sufficient intensity at a greater distance 



FTG. 28. INCANDESCENT 

 LAMP; MOUNTED. 



from the lamp, may also 'be obtained by sil- 

 vering one side of the glass bulb of the lamp 

 so as to reflect toward one side in a beam of 

 parallel rays, as shown in Fig. 29, the light fall- 

 ing upon it from the loop C, which is approxi- 

 mately in its principal focus. These lamps are 

 of a size and form made by Miiller, of Ham- 

 burg, from specifications by Dr. Theodor Stein, 



FIG. 29, INCANDESCENT LAMPS. 



of Frankfort- on -the-Main. They are supplied 

 with horseshoe carbons supported by two plati- 

 num wires, and should be used with caution, 

 as a slight excess of intensity in the current 

 will sometimes destroy the carbon.* ("A. M. 

 M. J.," 1884, p. 222; u Journ. N. Y. Mic. So- 

 ciety," 1885, p. 1). 



A far better mounting than that heretofore 

 mentioned is to attach the lamps, after the 

 plan introduced by Mr. Edward Bausch, to the 

 swinging tail-piece in stands of the Ameri- 

 can type, as shown in Fig. 30, in which case a 

 single lamp and mounting is sufficient, as it 

 can be readily slipped to any required distance 

 from the object, and as readily located at any 

 angle from the optical axis, either below or 

 above the stage.f The separate sub-stage bar, 

 which is shown in the cut, unused and turned 

 directly above the stage, should be turned 

 down and made to carry a thin iron shield 

 between the incandescent light, wherever it 

 may be placed, and the eye; also a porcelain 

 or opal glass, or white paper shade to be inter- 

 posed between the lamp and the object for use 

 with low powers, the narrow line of intense 

 illumination furnished by the filament being 

 favorable for definition and resolution with 

 high powers, but furnishing with difficulty the 

 white - cloud effects often desired with low 

 powers having a large field of view.J In using 



* To secure the warm-stage effects frequently necessary 

 in chemical and biological research, Dr. Stein places a coil of 

 platinum wire, capable of becoming heated by an electric cur- 

 rent, beneath the stage, for the purpose of heating the air 

 which passes upward through the stage-opening to the object ; 

 the temperature being indicated by a spiral bimetallic ther- 

 mometer, and controlled by regulating the current by the 

 rheostat. 



t Dr. Yan Heurck places the lamp in a box, with an open- 

 ing through the cover for the emission of light directly, with- 

 out reflection, to the object. 



\ For simpler stands having no separate sub-stage bar, 

 these accessories may be attached with somewhat less conve- 

 nience to the bar which carries the lamp itself. 



