Mirror and Mirror-Bar.- -The proper illumina- 

 tion of an object is an important feature and 

 although there are numerous accessories for 

 properly accomplishing- this, which will be spoken 

 of later on, the mirrors alone are effective agents 

 when properly constructed and applied, particu- 

 larly when no high magnification is used. The 

 plane mirror is generally . used with very low 

 powers and reflects light in about its original inten- 

 sity. The concave mirror, however, is intended to 

 concentrate the light so that all the rays which 

 strike its surface are reflected and come together 

 at some point above and the rays from the surface 

 being contained within a comparatively small 

 space, cause an increased intensity. This point is 

 called the focal point and is usually arranged to 

 coincide with the opening of the stage, when 

 parallel rays such as from the sky are used. When 

 the source of light comes considerably nearer to 

 the mirror, as for instance from a lamp and the 

 rays are diverging, the focal distance becomes 

 considerably longer. Some of the intensity is lost 

 in consequence, as well as the degree of conver- 

 gence. For this reason some mirror-bars are so 

 arranged that the distance of the mirror from the 

 stage may be varied to accommodate the variation 

 in the location of the source of light. While this 

 is of considerable aid, there is in some instruments 

 not sufficient room for a complete accommodation, 



