108 PBACTICAL PHOTO-MICBOGBAPHY 



this is interposed between the source of light and the sub-stage 

 condenser of the microscope, the ground-glass end being towards 

 the radiant. The rod may conveniently be made of about 

 half an inch or less in diameter, and of a length from four to 

 six inches. It may be placed quite close to the radiant, since 

 from its form it is particularly tolerant of any considerable 

 rise of temperature, and the glass is sufficiently conducting to 

 prevent risk of fracture or of fusion. By varying the distance 

 between the ground-glass end of the rod and the radiant, any 

 desired variation in the intensity of illumination may be 

 obtained. Evidently, the intensity of illumination will vary 

 inversely as the square of the distance between the filament 

 and the ground-glass surface. The light which in this way is 

 emitted from the ground-glass surface is transmitted by total 

 internal reflection almost in its entirety along the length of the 

 glass rod, with the result that the illumination is even and 

 uniform at whatever distance the speculum is from the radiant. 

 In effect therefore the speculum acts as a source of light, which 

 has none of the disadvantages of the original illuminant. The 

 apparatus in a convenient and perfected form may be obtained 

 from Messrs. Beck of Cornhill, E.G. 



For visual purposes this appliance is of very considerable 

 use ; but for photographic work, where usually some method 

 of broadening the image of the source of light by using collect- 

 ing lenses between the radiant and the sub-stage condenser is 

 employed, its use is not, perhaps, quite so necessary. But it 

 may be adopted with advantage where carbon or metallic 

 filament electric lamps are the only sources of light available. 



Sunlight. Direct sunlight may be necessary for some classes 

 of work, but with it the use of the heliosiat is advisable, if 

 not absolutely necessary. Any type in which the clockwork 

 movement is accurately and carefully made can be adopted. 

 It is necessary for it to be adjusted for the particular latitude 

 in which the work is being carried out. In this country it can 

 hardly be regarded as a satisfactory method of illumination 

 and will probably rarely be adopted. It introduces considerable 

 difficulty in estimating exposures owing to the variability of 

 the actinic value of sunlight. It must be admitted, however, 

 that in the early days of photo-micrography it was necessary, 

 and good results were obtained ; but the photographic plates 



