CH. II.-] LIGHTING AND FOCUSING. 35 



the objects are mostly opaque. In Vertebrate Histology, reflected light is but 

 little used ; but in the study of opaque objects, like whole insects, etc., it is used 

 a great deal. For low powers, ordinary daylight that naturally falls upon the ob- 

 ject, or is reflected or condensed upon it with a mirror or condensing lens, answers 

 very well. For high powers and for special purposes, special illuminating appa- 

 ratus has been devised (? 26). (See also Carpenter-Dallinger, p. 278). 



I 60. 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. 



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 

 a condenser (<S 75), and thence transmitted to the object from below (Figs. 48-51). 



\ 61. 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 illu- 

 minated. 



(S 62. Oblique Light. — This is light in which parallel rays from a plane mirror 

 form an angle with the optic axis of the microscope (Fig. 40). 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 (Fig. 49). 



DIAPHRAGMS. 



\ 63. 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 diaphragm, in 

 general, is to cut off all adventitious light and thus 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. The diaphragms of a condenser serve to vary 

 its aperture to the needs of each object and each objective. 



\ 64. Size and Position of Diaphragm Opening. — When no condenser is used 

 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 from 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 prevents 

 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. On the other hand, even with a 

 small diaphragm, the whole field will be lighted. 



With an illuminator or condenser (Figs. 41, 48), the diaphragm serves to narrow 



