36 LIGHTING AND FOCUSING [CM. II 



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 

 object, 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 

 apparatus has been devised (\ 28). (See also Carpenter-Dallinger, Ch. IV). 



\ 64. Transmitted Light. — By this is meant light which passes through an 

 object 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 [\ SS), and thence transmitted to the object from below (Figs. 48-51). 



\ 65. 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 coincident with 

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

 illuminated. 



\ 66. 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. 40). 



DIAPHRAGMS 



\ 67. 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 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. 



\ 68. 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. 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 considera- 

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

 the object and thus injures the distinctness of the microscope 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. On the other hand, even 

 with a small diaphragm, the whole field will be lighted. 



