514 



MICROSCOPY. 



Abbe condensers having lenses of moderate 

 size. Blue and ground-glass caps, center-stops, 

 polarizing prisms, or any special stops desired, 

 may be adapted at will. Among the special 



Fie. 25. IBIS ILLUMINATOR. 



stops used is the horizontal slit or pair of hori- 

 zontally arranged apertures,. Fig. 26, for the 

 better illumination of binocular microscopes 

 ("American Naturalist," 1870, p. 636). 



For dark-field illumination, the immersion 

 hemisphere with light, suitably condensed by 

 concave mirror or by lenses, passed through it 

 at an angle exceeding the semi-aperture of the 

 objective, is available on any stand. The Abbe 

 condenser with center-stops gives more light, 

 and leaves little to be desired. 



For opaque illumination, the swinging tail- 

 piece now allows the concave mirror to be 

 brought above the stage ; and in stands which 

 lack this facility the mirror is sometimes so 

 mounted as to be transferable to the stage it- 

 self for the same purpose. It is, however, not 

 fully equivalent to the bull's-eye. The "ver- 

 tical " illuminator for high powers of the 

 Messrs. Beck, apparently the most used of the 

 various devices for making the objective its own 

 condenser, is improved by the addition of dia- 

 phragms to limit the side aperture in its adapter 



Fie. 26. STOPS FOR BINOCULAR. 



through which the light enters to be reflected, 

 by an inclosed cover-glass, downward through 

 the objective to the object. Fig. 27 shows this 

 illuminator with cylindrical diaphragm, made 

 by James \V. Queen & Co., from designs by 

 Mr. Tilghman. Either of the small apertures 

 in its center ring can be brought into various 

 positions over the main opening, while the 

 whole ring can be made to cut off a vertical 

 half or more of the same. Tolles's internal 

 illuminator, shown in Fig. 18 in connection 

 with the "duplex" objective, being a reflecting 

 prism, projecting between the lower and mid- 

 dle elements of the objective system, and thus 





avoiding the glare caused by reflection from 

 the back lenses, has been lately revived by 

 Prof. W. A. Rogers, of Cambridge, Mass., and 

 been used to illuminate metallic surfaces, in 

 his elaborate series of researches in regard 

 to the comparative values of graduated 

 standards of length. It has also been used 

 for like purpose by M. Tresca, at the French 

 section of the International Bureau of 

 Weights and Measures at Paris. 



In monochromatic illumination, a substi- 

 tute for the rather inconvenient (and only 

 blue) ammonio-cupric cell is made by Mr. 

 Zeiss, at the suggestion of Prof. Abbe. It 

 consists of a tube, to be placed horizontally 

 beneath the stage, in which are a pair of 

 spectroscope prisms, which bend the illumi- 

 nating rays upward in the form of a spec- 

 trum through the stage opening. An ad- 

 justable slit permits the selection for use of 

 any desired color (" A. M. M. J.," 1883, p. 171). 

 A very simple and practical arrangement for 

 every-night use, in place of the ordinary blue 

 glass cap, is the graduated disk, contrived by 

 Mr. George H. Hopkins, and made by the 

 Bausch & Lomb Op- 

 tical Co., which re- 

 volves like a wheel, 

 and thus presents over 

 the well -hole inter- 

 mediate shades from 

 white to dark blue. 



In the improvement 

 of lamps for the mi- 

 croscope, much inge- 

 nuity has been ex- 

 pended. Gas and can- 

 dles having wholly 

 given place to petro- 

 leum as the source of 

 light, the cylindrical 

 Argand flame has been likewise discarded 

 Tolles having shown that the simplest lamp, 

 with the edge of its fiat wick turned toward 

 the stage or mirror, gave an easier and clearer 

 resolution than the best "student" lamp. For 

 the large shades surmounting the whole lamp, a 

 cylinder of sheet-iron, or blackened brass or tin 

 just surrounding the chimney, is substituted, 

 With an opening at one side, at the level of the 

 flame, for the emission of light. The opening 

 is often round and supplied with diaphragms, 

 or with a bull's-eye, in a cylindrical fitting, to 

 be focused upon the edge of the flame. Lat- 

 terly, the glass chimney has frequently been 

 dispensed with, and the metallic cylinder, with 

 its inserted bull's-eye, made to take its place ; 

 or, the bull's-eye being not always required, 

 the opening in the cylinder is closed by a 3 x 1 

 glass object-slide slipped into grooves at its 

 edges, or by a similar slip of blue or opal glass, 

 the bull's-eye being mounted upon an adjacent 

 arm of its own. For great portability, the 

 pocket-lamps are necessarily made with very 

 small oil -vessel and correspondingly small 

 wicks ; but for other purposes the latest lamps 



FIG. 27. VERTICAL ILLUMI- 

 NATOR. WITH CYLINDRICAL 

 DIAPHRAGM. 



