Illumination and , Illuminating Apparatus 87 



and that alone utilized in the microscope. The use of a 

 heliostat is necessarily limited in Great Britain, and 

 especially so in London, on account of the few hours of 

 bright sunlight that are available, but in countries where 

 this restriction does not apply the heliostat is particularly 

 to be recommended, not only for this especial purpose, but 

 also for general microscopical work and in photography. 



Many experiments have been made with a view to pro- 

 ducing monochromatic light screens by means of pigments 

 and the combinations of coloured glasses, and thereby 

 obviating the necessity for prisms. So far these attempts 

 have been only partially successful, all that have been made 

 passing light of more than one colour, but it is to be hoped 

 that the desired result may yet be achieved. Very material 

 assistance is, however, afforded by these screens, not alone 

 for actual monochromatic illumination, but also for general 

 work, for they allow of a large cone of illumination being 

 utilized without discomfort to the eyes. They are usually 

 placed beneath the sub- stage condenser, and are employed 

 in photo-micrography very considerably for neutralizing 

 non-photographic colours in objects, and rendering the 

 actinic and visual rays in an objective more nearly coin- 

 cident, also in visual work for minimizing light glare with 

 large cones of illumination. This latter applies equally to 

 apochromatic and achromatic objectives, the screen often 

 producing stronger contrast and a crisper image than could 

 be obtained without one. 



The most advantageous colour for all-round visual purposes 

 is the green-blue, and excellent light screens are made on a 

 plan, originally suggested by Mr. J. W. Gifford, consisting of 

 a film of gelatine stained with malachite green deposited on 

 a circular disc of signal green glass, and having a protect- 

 ing cover-glass for the gelatine fixed by means of a ring 

 of cement. Fluid screens can also be made with great 

 accuracy, and with care will last a considerable period, but 

 it is particularly essential that they be protected from light, 

 or they are liable to fade. The Gifford's screen referred to 



