i6 MICROSCOPE AND ACCESSORIES. 



densing lens, answers very well. For high powers and for special purposes, 

 special illuminating apparatus has been devised (Fig. 12). (See Carpenter). 



\\\. Transmitted Light.— By this is meant light which passes through an ob- 

 ject from the opposite side. The details of a photographic negative are in many 

 cases only seen or best seen by transmitted light, while the print made from it is 

 best seen by reflected light (§40). 



Almost all objects studied in Vertebrate Histology are lighted by transmitted 

 light, and they are in some way rendered transparent or semi-transparent. The 

 light traversing and serving to illuminate the object in working with a compound 

 microscope is usually reflected from a plane or concave mirror, or from a mirror 

 to an illuminator (\ 54), and thence transmitted to the object from below (PI. 

 II, Fig. 10, 13 ; PI. Ill, Fig. 20). 



f 42. Axial or Central Light. — By this is understood light reaching the object, 

 the rays of light being parallel to each other and to the optic axis of the micro- 

 scope, or a diverging or converging cone of light whose axial ray is parallel with 

 the optic axis of the microscope. In either case the object is symmetrically 

 illuminated. 



\ 43. Oblique Light. This is light in which parallel rays from a plane mirror 

 form an angle with the optic axis of the microscope (PI. II, Fig. 12, 13 c). Or 

 if a concave mirror or a condenser is used, the light is oblique when the axial 

 ray of the cone of light forms an angle with the optic axis (PI. Ill, Fig. 20 a ). 



DIAPHRAGMS. 



$ 44. Diaphragms and their Proper Employment. — Diaphragms are opaque 

 disks with openings of various sizes, which are placed between the source of light 

 or mirror and the object. In some cases an iris diaphragm is used, and then the 

 same one is capable of giving a large range of openings. The object of a dia- 

 phragm, in general, is to cut off all adventitious light and thus to enable one to light 

 the object in such a way that the light finally reaching the microscope shall all 

 come from the object or its immediate vicinity. 



\ 45. Size and Position of Diaphragm Opening.— The size of the opening in the 

 diaphragm should be about that of the front lens of the objective used. For 

 some objects and some objectives this rule may be quite widely departed from ; 

 one must learn by trial. 



When lighting with a mirror the diaphragm should be as close as possible to the 

 object in order, (a) that it may exclude all adventitious light from the object ; (b) 

 that it may not interfere with the most efficient illumination by the mirror by 

 cutting off a part of the illuminating pencil. If the diaphragm is a considerable 

 distance below the object, (1) it allows considerable adventitious light to reach 

 the object and thus injures the distinctness of the microscopic image; (2) it pre- 

 vents the use of very oblique light unless it swings with the mirror ; (3) it cuts off 

 a part of the illuminating cone from a concave mirror (Fig. n). 



With an illuminator (PI. Ill, Fig. 20), the diaphragm serves to narrow the pen- 

 cil to be transmitted through the condenser, and thus to limit the aperture 

 or for any special purpose to be served (see \ 61). Furthermore, by 

 making the diaphragm opening excentric, oblique light may be used, or by using 

 a diaphragm with a slit around the edge (central stop diaphragm), the center re- 

 maining opaque, the object may be lighted with a hollow cone of light all of the 

 rays having great obliquity. In this way the so-called dark-ground illumination 

 may be produced {\ 60; PI. Ill, Fig. 20). 



