4 6 



LIGHTING WITH THE MICROSCOPE 



[Ch. II 



reflected light is but little used; but in the study of opaque objects, 

 like whole insects, etc., it is used a great deal. For a simple micro- 

 scope and low powers of the compound microscope, ordinary day- 

 light that naturally falls upon the object, or is reflected or condensed 



upon it with a mirror or condensing 

 lens, answers very well (fig. 21 A, 34). 

 For high powers and for special pur- 

 poses, special illuminating apparatus 

 has been devised (fig. 50). 



§ 85. Transmitted light. — By this 

 is meant light which passes through 

 an object from the opposite side (fig. 

 21 B, 35). The details of a photo- 

 graphic 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 (fig. 21 A, 



34)- 



Almost all objects studied in ani- 

 mal and vegetable Histology are 

 lighted by transmitted light, and 

 they are in some way rendered trans- 

 parent or semi-transparent. The 

 light traversing and serving to illu- 

 minate the object in working with 

 a compound microscope is usually 

 reflected from a plane or concave 

 mirror, or from a mirror to a con- 

 denser, and thence transmitted to 

 the object from below (fig. 20, 41). 

 § 86. Axial or central light. — By 

 this is meant light reaching the object in such a way that it is sym- 

 metrically arranged around the optic axis of the microscope, then the 

 object will be equally illuminated from all sides. If bundles of paral- 

 lel rays are reflected upon the object from the mirror, they must be 

 so disposed that the object will receive an equal quantity of light 



E3 



2ta.ge 



Fig. 34. Low-power Objec- 

 tive Showing Working Dis- 

 tance and Reflected Light. 



Axis The principal optic axis 

 of the objective extended. 



SI The glass slide on which the 

 object is mounted. 



O Object. 



c Cover-glass over the object. 



W The working distance be- 

 tween the cover and the objective. 



Mirror The mirror is repre- 

 sented as above the stage and re- 

 flecting parallel beams upon the 

 object. 



FC Front combination of the 

 objective. 



BC Back combination of the 

 objective; it is composed of a 

 plano-concave of flint (F) and a 

 double convex lens of crown glass 

 (c). 



