512 



MICROSCOPY. 



choices. For convenience in interchanging 

 ocular and snb-stage accessories, 0*75 inch was 

 recommended as a size for the former and 1-50 

 for the latter (" Proc. Am. S. M.," 1884, p. 

 228). An ideal series of oculars would proba- 

 bly be 1-00, 1-25, and T40 inch, but the com- 

 mittee were constrained to adopt the 1'35 inch 

 as the largest, though too near the T25, in 

 order to avoid the confusion of too many stand- 

 ards, the Royal Microscopical Society, after 

 the appointment of the American committee 

 had been made public, and co-operation with 

 its objects had been solicited, having selected 

 and published sizes of its own (of which 1'35 

 was the largest) without conference with the 

 society which had opened the subject and was 

 known to be working upon it. It was also 

 recommended and decided to name oculars, 

 like objectives, by their equivalent focal length 

 in inches. This approximate system, claiming 

 not extreme accuracy but much convenience, 

 has been from the first applied to objectives 

 both in England and America, and has been 

 likewise applied with satisfaction to the nam- 

 ing of oculars by a few makers and by many 

 microscopists. In order to make the name 

 suggestive of the actual working value of the 

 ocular, so that, for example, a one-inch ocular 

 could be substituted for a two-inch for the 

 purpose and with the effect of doubling the 

 power, it was decided, at least temporarily 

 until experience should develop a better plan, 

 to estimate the numerical names on the con- 

 ventional basis of one-inch focus, representing 

 an amplification of ten diameters as actually 

 used in the compound microscope having some- 

 what arbitrarily chosen tube -length of ten 

 inches, including the objective (see tube- 

 length, p. 502, supra). 



Illumination. In illuminating apparatus, the 

 more indispensable varieties of which are con- 

 sidered less as accessories than as parts of the 

 stand itself, two important and radical inno- 

 vations have appeared, the swinging tail-piece 

 above described, and the immersion system. 



Only less important than these are the 

 changes in the construction and arrangement 

 of diaphragms for limiting the amount of light 

 received from the mirror or condenser. The 

 conventional wheel of apertures, a crude and 

 clumsy arrangement which 

 had been typical for years, 

 has been replaced by the 

 graduating diaphragm, of 

 which the smaller forms are 

 called " iris " diaphragms. 

 The recent modifications of 

 this by Sidle and Wale and 

 Bausch & Lomb (see Fig. 20) 

 ______ are small and cheap enough 



FIG. 20. IRIB DIA- f r anv stand. Such dia- 



PHBAGM. phragms are, in the writer's 



judgment, by far the most 



useful and perfect means of accomplishing the 



object for which they are designed. For very 



small stands, as the Zentmayer " Histological," 



the efficient Continental expedient of a " cylin- 

 drical" diaphragm, a dark well with a variety 

 of perforated caps, is sometimes substituted. 

 Like the "iris," it is mounted on a sub-stage 

 or in a sliding tube, so that its position may 

 be varied from close to the object-slide to a 

 considerable distance below it. As increase of 

 distance diminishes proportionally the cone of 

 light transmitted from the concave mirror, 

 some of the advantages of the "iris" are thus 

 secured to a limited extent. The cylinder is 

 sometimes contracted above into a cone taper- 

 ing to the edge of the small aperture, so that 

 the diaphragm may be swung along with the 

 mirror, as it often must be when there is but 

 one tail-piece, without striking against the ob- 

 ject-slide (see cut of " Histological " stand). 

 The Continental fashion of letting a plate ot 

 apertures into the stage, as near as possible 

 to the bottom of the object-slide, has been 

 adopted by a few American and English mak- 

 ers; but, while very effective in excluding 

 stray light, it falls far short of the above as a 

 means of controlling the angular breadth of 

 the illuminating pencil. 



Diaphragms with small apertures, or con- 

 densers with small lenses, can be used to real- 

 ly good advantage only with a centering ad- 

 justment. In the absence of a centering sub- 

 stage, a centering nose-piece may easily be 

 made to do duty below the stage, as suggested 

 by Mr. E. M. Nelson (" J. R. M. S.," 1881, p. 

 126). 



The use of immersion illuminators was an 

 inevitable and early sequence to that of the 

 corresponding objectives, and it has, at the 

 same time, simplified and improved the illu- 

 mination of dry objectives. The simplest and 

 one of the most useful forms, is the hemi- 

 spherical lens (including the average thick- 

 ness of the glass object-slide) connected with 

 the slide with glycerine or oil in such manner 

 that the object shall be at its center of curva- 

 ture. This device, introduced for a compara- 

 tively trivial purpose by Mr. Wenham in 1856, 

 was assigned to practical use by Mr. Tolles in 

 1872, and is now universally adopted. It is 

 applicable to any microscope however meager, 

 and is of such almost universal utility, espe- 

 cially in combination with the swinging tail- 

 piece, that a person experienced in its use for 

 general work often finds little necessity for any 

 other illuminator. Its condensing power is 

 small, the object being so far within its focus, 

 but it is used as a means of passing light, with- 

 out deflection of the axis of the pencil, and with 

 a peculiarly clear and brilliant effect, directly 

 from the mirror or source of light, in what- 

 ever position it may be placed, to the object. 

 The interior " balsam " angle of the illumina- 

 tion is therefore directly adjusted by the swing 

 of the tail-piece and indicated by its graduations 

 (shown in engraving of "arc" stand, Fig. 12). 

 When more condensation is required, an objeo 

 tive of one to two-inch focus is inserted in 

 the sub-stage between the mirror and the 



