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 



£ta.g<> 



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. 



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). 



