ILLUMINATING APPARATUS 37 



the fluid containing the object, as reflections from dirt or bubbles 

 may cause a glare that destroys the black background against 

 which the illuminated objects stand out. 



A strong source of light for use with this illuminator is essential, intensity 

 It is referred to under the heading of " Illuminants." If the°^"=^^- 

 light is of only moderate intensity, a bull's-eye condenser should 

 be used. It should be placed at such a distance that an image 

 of the lamp is formed in the centre and on the surface of the 

 mirror. The correct position is best ascertained by holding a 

 white card on the mirror while the bull's-eye is adjusted between 

 the lamp and the mirror. 



When a colour filter is used, a stronger light than would other- 

 wise be required should be available. The light, however, must 

 not be too strong, for although a weak light will not illuminate 

 such transparent structures as bacteria sufficiently to render 

 them visible, too strong a light shows up certain diffraction 

 images and destroys definition. 



It is, therefore, necessary to reduce the illumination to just 

 such an extent that these diffraction effects are not aggressively 

 apparent. 



A very brilliant source of illumination, such as an electric 

 " Pointolite " lamp, gives more light than is required, except for 

 use with colour filters ; but used in combination with the adjustable 

 neutral tint wedges (p. 32), gives every intensity of illumination 

 that is required for this and all other classes of microscopic work. 



Those who have not used this form of illumination cannot Resolution 

 realise the large amount of extra structure that can be recognised ^^^^^^ ^^^J^^^ 

 by its means in certain classes of objects. The spines or illumination, 

 pseudopodia of Coscinodiscus were discovered by dark-ground 

 illumination. Hidden structure in bacteria has been revealed, 

 and the markings of diatoms are shown with greater brilliancy 

 by this illuminator than by any other means. The resolution of 

 diatoms is much easier with dark-ground illumination because it 

 gives far greater contrast. If an object is not opaque and only 

 appears slightly darker than the background, it is but faintly 

 seen when viewed by transmitted light ; but if it has even a small 

 power of reflecting light it can be made to show brilliantly upon 

 a black ground provided the illuminating source is sufficiently 

 powerful. 



Reflection of light takes place from any object that has a 

 different refractive^ index or density from that of the material in 

 which it is situated. A very small difference of density only is 

 required to give reflection. An interesting experiment to illus- 

 trate this consists of taking two plates of glass with a layer of 

 water between them, and flowing in from one side a few drops of 

 a highly refracting fluid, which gradually mix with the water 

 and raise its density above that of glass. As the fluid mixes, 

 the density is gradually raised from point to point, and at the 



