MICEOSCOPY. 



513 



hemisphere, in which case, as proposed by Mr. 

 Bulloch, the mirror is sometimes replaced by a 

 small lamp mounted on the tail-piece to furnish 

 light without reflection. 



When the hemisphere is held against the 

 elide only by the interposed glycerine or oil, 



FIG. 21. PRISM (OB LENS) HOLDER. 



it is liable to slip out of place, especially if the 

 fluid be too abundant or the stage be inclined. 

 To prevent this, a slide is sometimes prepared 

 by pasting a paper slip on the back as a ledge 

 for the lens to rest against, or by forming a 

 cement ring of suitable size and location to 

 surround it when in use; and for the same 

 purpose the hemisphere is sometimes cemented 

 permanently with balsam to the bottom of a 

 glass slide, or is mounted in the center of a 

 thin brass plate fitted to the well-hole of the 

 stage. In all these cases the centering of the 

 lens below the object is somewhat difficult or 

 imperfect, and a better method is to support it 

 by a wire attached to the sub-stage, or, where 

 the sub-stage is wanting, to a special holder 

 which, like the ingenious device of Queen & 



FIG. 22. STEPHENSON'B CATOPTRIC ILLUMINATOR. 



Co. (Fig. 21), can be clamped to any stage. 

 Even if provided with a sub-stage the sepa- 

 rate holder is sometimes useful, in case the 

 sub - stage is required to carry a condensing 

 objective. For small lined objects under high 

 powers the "Wenham button" (Carpenter, 

 "The Microscope," p. 123), whose sharper 

 curvature has its focus nearer 

 the object, and which therefore 

 acts more efficiently as a con- 

 denser, is often employed and 

 may be similarly mounted. The 

 immersion prism, shown in situ 

 on the holder in Fig. 21, and 

 popularly called the Woodward 

 prism, though introduced to mi- 

 croscopy years before by Mr. 

 Wenhain, is of very limited use compared with 

 the hemisphere. Stephenson's catoptric illu- 

 minator is a plano-convex lens whose flat side 

 has a silvered center to reflect from above as 

 well as to stop out axial rays, and whose con- 

 vex surface is silvered above for reflection, and 

 VOL. xxiv. 83 A 



flattened at the center for immersion contact 

 with the object-slide, as shown in Fig. 22. 

 With dry objectives, opaque and dark -field 

 effects are of course produced according as the 

 object is in balsam or dry on the slide (" A. M. 

 M. J.," 1880, p.^204). Perhaps the simplest 

 means of producing similar results is a slip of 

 common looking-glass lying under the slide, 

 with immersion contact, and reflecting up- 

 ward the rays received from above, somewhat 

 as in Fig. 23 ; the ray, H, meeting the slide, 8, 

 is reflected by the mirror, R, to the object at 

 F, beneath the objective, B. Sometimes an 

 immersion prism is used as a means of getting 

 the ray, H, into the slide without refraction, 

 or H is made to enter a beveled end of the slide 

 for the same purpose. Variations upon this 

 really efficient plan have been described by 

 numerous writers ( k 'Am. Nat.," 1871, p. 607: 

 1876, p. 752 ; " A. M. M. J.," 1880, p. 205 ; 1884, 

 p. 102). 



If the immersion illuminating lens be thick- 

 ened to more than a hemisphere, and fur- 

 nished below with one or two collecting lenses 



c R 

 FIG. 23. LOOKING-GLASS OBLIQUE ILLUMINATOR 



to increase the amount of light and give it such 

 convergence as to come to a focus upon the 

 object, it becomes the "Abbe illuminator," 

 which is now made by various opticians, and 

 is largely taking the place of all other sub- 

 stage condensers. Two such combinations sre 

 shown in section in Fig. 24, giving numerical 

 apertures of 1-20 and 1-40 respectively; and, 

 from present experience, they seem preferable 

 for illuminating purposes to the most carefully 

 corrected achromatic condensers. 



In the so-called " iris illuminator " (" Pn>c. 

 A. S. M.," 1884. p. 160), instead of the revolv- 

 ing wheel or sliding plate with various sizes of 

 apertures, hitherto combined with the con- 

 densing lens systems, the writer has intro- 

 duced the iris diaphragm for that purpose, the 

 iris being mounted, as shown in Fig. 25, in a 



FIG. 24. ABBE CONDENSERS. 



sliding plate, which gives it a decentering ad- 

 justment, and utilizes its perfect control of 

 light, not only for central light, but also for 

 pencils of any desired obliquity. This arrange- 

 ment seems specially applicable to the four- 

 tenths achromatic condenser, or to any of the 



