1 9 MICRO CSOPE AND ACCESSORIES. 



CENTRAL AND OBLIQUE LIGHT WITH A MIRROR. 



§ 52. Axial Light, (§ 42).— Place a preparation containing minute 

 air-bubbles under the microscope. (The preparation may be easily 

 made by beating a drop of mucilage on a slide and covering it. See 

 Ch. II). Use a }4 inch (3 mm.) or No. 7 objective, and a medium 

 ocular. Remove all the diaphragms and the sub-stage. Focus the 

 microscope and select a very small bubble, one whose image appears 

 about 1 mm. in diameter, then arrange the plane mirror so that the 

 light spot in the bubble appears exactly in the center. Without chang- 

 ing the position of the mirror in the least, replace the air-bubble pre- 

 paration by one of Pleurasigma angulatum or some other finely marked 

 diatom. Study the appearance very carefully. 



§ 53. Oblique Light, (§ 43). — Swing the mirror far to one side so 

 that the rays reaching the object may be very oblique to the optic axis 

 of the microscope. Study carefully the appearance of the diatom with 

 the oblique light. Compare the different appearance with that of central 

 light. The effect of oblique light is not so striking with histological 

 preparations as with diatoms. 



It should be especially noted in §§ 52, 53, that one cannot determine 

 the exact direction of the rays by the position of the mirror. This is 

 especially true for axial light, (§ 52). To be certain that the light is 

 axial some such test as that given in § 52 should be applied. (See also 

 Ch. II, under Air-bubbles). 



ABBE ILLUMINATOR OR CONDENSER. 



§ 54. For all powers, but especially for high power objectives, a con- 

 denser or illuminator is of great advantage. The one most generally 

 useful was designed by Abbe. It consists of two or three very large 

 lenses which are placed in some form of mounting beneath the stage. 

 It serves to concentrate a very wide pencil of light from the mirror 

 upon the object. For the best work in modern histology the Abbe il- 

 luminator is almost as indispensable as the homogeneous immersion 

 objectives (PI. Ill, Fig. 20). 



§ 55. Centering and Arrangement of the Illuminator. — The 

 proper position of the illuminator for high objectives is one in which 

 the beam of light traversing it is brought to a focus on the object. If 

 parallel rays are reflected from the plane mirror to it, they will be 

 focused only a few millimeters above the upper lens of the illuminator ; 

 consequently the illuminator should be about on the level of the top of 

 the stage and therefore almost in contact with the lower surface of the 

 slide. For some purposes, when it is desirable to avoid the loss of light 

 by reflection or refraction, a drop of water or homogeneous immersion 



