CH. II] 



LIGHTING AND FOCUSING 



special purposes, special illuminating apparatus has been devised 

 also Carpenter-Dallinger, Ch. IV. ) 



31). 



41 

 (See 



\ 73. Transmitted Light.— By this is meant light which passes through 

 an object from the opposite side. The details of a photographic negative 



Stig 



44 

 Figs. 44-45- For full explanation see Figs. 27 and 28. 



45 



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 trans- 

 mitted light, and they are in some way rendered transparent or semi-trans- 

 parent. 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 ($ 99), and thence transmitted to the 

 object from below (Figs. 54-57). 



I 74. — 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 microscope, or a diverging or a converging cone of light whose axial ray 

 is coincident with the optic axis of the microscope. In either case the object 

 is symmetrically illuminated. 



§ 75. Oblique Light. — This is light in which parallel rays from a plane 

 mirror form an angle with the optic axis of the microscope (Fig. 45). 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. 45). 



