114 



PHOTO -MICROGRAPHY 



sunlight or electric light be used, the specimen cannot be seen with sufficient clearness 

 to be properly focussed. 



The next best arrangement to obtain monochromatic light is by the use of a 

 trough made entirely of glass, the vessel having parallel sides, and being filled with 

 solutions of certain strength and different substances, according to the result required. 

 These are held in situ by the Biinsen holder. Zeiss makes excellent troughs for this 

 purpose, but they are very expensive. English firms now supply them cheaper and 

 we do not know they are in any way inferior. Further reference will be made to 

 these matters when dealing with specimens requiring either complete or partial one- 

 coloured lig-ht. 



9. Polarised Light. — It is not the place here to discuss or explain what is 

 meant by the polarisation of light interesting as it undoubtedly is, and so it will only 

 be spoken of so far as the subject afiects the photo-micrographer. 



It is obtained for his purpose by the use of two Nicol's prisms which are named 

 after their inventor. One is placed beneath the object in the substage, and is eitlier 

 capable of revolving on its axis by its own special arrangement, or being fixed into 

 the body of the substage, is made to revolve by a special construction to be found 

 ready made in that part of the microscope itself as built by several makers. Any- 

 how (and no matter by what means) it must be capable of revolution about its axis. 

 The second Nicol is placed either in the body of the tube — the most common place — 

 just above the objective ; or is mounted with lenses as an eye-piece and placed at 

 the eye end of the tube instead of the ordinary occular.* Revolution of either about 

 its axis produces the effect as seen down the microscope of intermitting waxing and 

 waning of light. To produce the well known and beautiful variations of colour a film 

 of selenite or of mica is interposed between these prisms. Revolution then causes 

 marvellous changes of colour. If now suitably prepared objects are placed on the 

 stage, examples of colour rendering are exhibited on rotating one or either prism too 

 beautiful to describe. With some specimens it is not needful to use the mica or 

 selenite for they themselves are able to produce excellent difierentiation in colour or 

 in varying phenomena in black and white. Both of these the operator can photograph. 

 The actual taking of the negative is no more difficult than any other. All sorts of con- 

 trivances are made for placing difierent thicknesses of selenite or of mica in the field but 

 for a description of these the reader is necessarily referred to books on the microscope, 

 (ii) How to ascertain how much an object is magnified. — It must be 



Owing to the material (Icelaiid spar) being so scarce, Zeiss now sells a prism smaller in size, after Prazmowski 

 vhich fits over an ordinary eyepiece. 



