CRITICAL ILLUMINATION 117 



the wick, instead of the flat, facing the microscope at 

 about 8" from the mirror. 



1. The use of the Mirror. The mirror has plane and 

 concave surfaces, of which the plane should be employed 

 whenever a condenser is used. To study the use of the 

 concave mirror, place a f" or 1" objective and low eye- 

 piece on the microscope, and take a suitable object. 

 Remove the condenser and illuminate with the mirror 

 only. It will be found that the concave gives a more 

 intense light than the plane side, because the rays that 

 it reflects are convergent instead of being parallel or 

 divergent. The mirror should be capable of a vertical 

 motion along the bar that carries it. Move it to or 

 from the object until the position is found in which 

 it gives the brightest possible light over a small portion 

 of the preparation. The focus of the mirror, or the point 

 at which it forms an image of the radiant, is then in the 

 plane of the object. It may be remarked that on account 

 of spherical aberration the mirror has no exact focus, 

 but the region is fairly definite. The illumination under 

 these conditions, though intense over a small area of the 

 object, is of little use for photomicrography, on account 

 of the image of the radiant which usually lies across the 

 field of view, and it is necessary to raise the mirror until 

 the object is evenly illuminated and the field just filled 

 with light. If the mirror is raised any higher a diminution 

 of light will be noticeable. This method of illumination 

 is sometimes satisfactory with low powers, but it is never 

 used when a suitable condenser is available. 



2. To Centre the Mirror. (a) Remove the objective 

 and condenser, and elongate the draw-tube to its full 

 extent. Look into the eyepiece from about 10" above it, 

 a small circle formed by its diaphragm is seen in the 

 centre of the eye-lens. Then move the mirror until 

 an image of the light source is seen exactly in the centre 

 of this circle. Use the flat side of the mirror. 



