158 PHOTOMICROGRAPHY 



dispense with them altogether in favour of gelatine and 

 glass. The strength of solution required naturally varies 

 very much with the thickness of the containing vessel 

 and the intensity of the illuminant. Metallic salts can be 

 dissolved in water until a saturated solution is obtained ; 

 the amount of solid that a certain quantity of water will 

 dissolve differs for each, but if a few crystals remain 

 at the bottom of the bottle, after due time has been 

 given for solution with frequent shaking, the solution is of 

 maximum strength. Dyes may generally be used in the 

 proportion of 1 part of the dye to 1000 parts of water. 

 These strong solutions are required in narrow cells up to 

 about a centimetre in width, they should be diluted for 

 use in larger vessels. 



Filter Diagram. A diagram of the absorptions and 

 transmissions of a few of the more important solutions is 

 given opposite. The unit of wave length used on the 

 scale is the ten-millionth of a millimetre, the Angstrom 

 unit, often called the " tenth metre." Wave lengths are 

 also expressed in micra (thousandths of a millimetre), /UL, 

 and in millionths of a millimetre, IUL/UL. The wave length 

 of the yellow D line of sodium may, for instance, be 

 expressed in any of the following ways, of which the first 

 is here adopted : 



X -5896-16 A.U. ort.m. 

 or 589-616 JUL/UL 

 or -589616 M 



or 5-89616x10-5 cm. 



The absorptions given are those obtained when a 

 Nernst lamp is used with solutions of the strengths 

 mentioned in cells of J" width. The only solution whose 

 preparation is not clear from the description given in the 

 table is the ammoniacal copper sulphate, and this is made 

 by adding sufficient strong ammonia to the saturated 

 copper sulphate solution to dissolve the precipitate at 



