MEDIUM-POWER PHOTO-MICROGRAPHY 



133 



Fig. 63 



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the right size not to cat off too much light from the reflector, and another that the 

 adjustments of the reflector shall be such as to just cause the light to fall evenly 

 and yet with a certain amount of difi'usion over the entire specimen. 

 Fig. 6, Plate V., was done by this means. 



A parabolic reflector, pb^ced on one side only of the specimen and 

 so arranged as to be illuminated from a jet placed on the other 

 side, is often of use with very low powers, but does not appear to be 

 very effective with any power above a 5 inch. 



Zeiss, Powell and Lealand, Watson, and some others also make 

 different forms of illuminators for opaque objects which are constructed 

 on the principle that a side light is so bent by a reflector or by a 

 prism as to pass through the objecKve on to the specimen, thence 

 back again through the objective to the eye. Personally we confess to 

 have had but little success with the arranp-ement : so far as we have 

 tried it we are not impressed by its utility. It is shown in Fig. 63. 



Arranging the Apparatus and taking a Photograph with 

 Polarised Light. — It has been explained on page 114 the general 

 detail of the arrangement. Seeing so much light is lost by polar- 

 isation, and seeing, too, that every angle- of-rotation of the nicol 

 on its .axis varies still more or still less the liglit which falls on the 

 plate, so it is impossible to furnish any more than an idea of 

 exposure. Very fine results can be obtained by the use of crystals 

 of numerous substances which have Ijeen dissolved in some solvent, dropped on 

 to a cover glass, dried and so crystallised and then dry mounted. These, although 

 not so pretty to the eye, photograph well without the selenite plate or the mica strip. 

 A short list is given in the footnote.* The negative should be developed very strongly 

 and very hard, to give the most striking contrast, and the exposure increased about 

 one-third : this varies every time the nicol is turned so it may require to be increased 

 to twice the usual amount. 



■ ty ■■■■ ■ ■■ _L Ulbiia J 



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The following make good specimens either with or without a selenite : 



Salicine 

 Bitartrate of potash 

 Salicylic acid 

 Sulphate of copper 

 Oxalic acid 



Phosphate of ammonia 

 Santonine 

 Fj'rogallic acid 



Asparagin 

 Chlorate of potash 

 Borax 

 Citric acid 

 Menthol 



Platino-cyanide of magnesia 



Cane sugar 



Thymol 



Sulphates of magnesium and copper 



Leg of a black beetle 

 A thin piece of horn 

 A slip of a quill pen 



A thin grinding of flint and some other rockg 



