ILLUMINATION OF OBJECTS; ILLUMINATING DEVICES 51 



Orthogonal illumination is employed in the study of colloids 

 and other particles in suspension in liquids and for the study of 

 particles in transparent or translucent solids, such as glass, etc. 

 For details as to apparatus and their use, see page 57. 



g. Differential Color Illumination by the method of Rhein- 

 berger 1 may be obtained by substituting for the dark-ground 

 wheel stop of the Abbe condenser colored disks of transparent 

 material, using a darker color for the central portion and sur- 

 rounding this disk with an annular ring of a lighter and strongly 

 contrasting color. The object will then appear strongly illu- 

 minated, but colored upon a colored background. If, for exam- 

 ple, the central disk is blue and the ring red, the objects will 

 appear red upon a blue background. With care and a suitable 

 choice of colors, very remarkable results may be obtained which 

 may greatly facilitate the study of certain sorts of material. 



h. By Means of Ultraviolet Light. When ultraviolet rays 

 impinge upon certain substances they become fluorescent and 

 glow with violet, red, green or bluish light. The color of the 

 fluorescence is peculiar to the substance. Since comparatively 

 few bodies exhibit this phenomenon and since the color is a 

 further aid in differentiation, advantage has been taken of this 

 property of bodies as a means of identification of such sub- 

 stances not readily recognized when present in low per cents hi 

 mixtures. To permit the extension of this method to minute 

 amounts of material the " Fluorescence Microscope" has been 

 constructed. 2 



Ordinary glass is practically opaque to ultraviolet rays but 

 not to the light rays resulting from the fluorescing of the sub- 

 stance; the ultraviolet rays however readily penetrate quartz. 

 We have, therefore, only to substitute quartz for glass in the 

 condenser in order to concentrate the ultra rays on the object 

 upon the stage. It follows from this that although the illu- 

 minating devices must be of quartz, as also the object slide upon 

 which the object lies, the objective and ocular may be those 

 ordinarily employed. 



1 J. Roy. Micro. Soc., 1896, 373; Spitta, Microscopy, London, 1909; 175-178. 



2 Made by C. Reichert, Vienna, Austria. 



