124 ELEMENTS OF MICROSCOPY. 



adjustments can be made without the position of the mirror being 

 changed. Imagining then that the rays from the microscope are 

 falling horizontally on the mirror, and that the microscope tube is 

 inclined to the table at an angle of 50^, it is evident that the 

 mirror must make an angle of 70° with the table in order that the 

 ray may be thrown fairly into the tube of the microscope. We 

 shall suppose the concave mirror to be used, and, whilst it is 

 certainly not the best method of lighting, it will probably be most 

 useful to the majority of microscopists if we indicate the method 

 of using it to the best advantage. 



A concave mirror has one principal focus, and the distance of 

 this may be easily found by allowing the rays of the sun to fall 

 upon the mirror, and noting the distance at which the image of 

 the sun is sharply formed upon a nar7'oiv slip of paper held 

 between the mirror and the sun. The slip must be narrow, in 

 order that it may not obstruct too much of the light which falls 

 upon the mirror, and should be opaque, in order that the image 

 may be clearly seen. When the proper point is found the paper 

 will be blackened by the concentrated heat rays. 



The rays from the sun being parallel, the image will be formed 

 at the principal focus of the mirror, which is always spoken of as 

 being of this focal length, and the length in question is half the 

 radius of the sphere, of which the mirror is a part. The sun not 

 being always available for the microscopist's purpose, an alternative 

 method may be useful. Setting up the lamp, with the flat side 

 of the flame toward the observer, a cardboard box is put by its 

 side, so that it is open toward him, with its bottom as far from 

 him as the flame is, the two being close together. The mirror is 

 now moved toward the lamp and box until the image of the flame 

 is sharply defined on the bottom of the box. This distance from 

 the mirror to the flame is the radius of curvature, and half the 

 distance is the principal focus, of the mirror. The reason for 

 this has to do with the first principles of optics, of which every 

 microscopist should know something, and we trust our readers will 

 follow us a little farther still. The point at which the image of 

 the lamp flame is formed, and the point at which the flame is 

 situated, are called conjugate foci. The nearer the flame is to the 

 mirror, the farther ofl" is the conjugate focus at which the image is 



