212 PKACTICAL PHOTO-MICBOGKAPHY 



stronger solution. Such a filter is very conveniently made 

 in the manner described by holding an ordinary rubber elastic 

 band with paper clips between two pieces of glass. A 

 thin rubber band will give approximately the thickness 

 mentioned. When using such a cell the dyes may be kept 

 as saturated solutions in water, and with the above thickness 

 will efficiently absorb the whole of the blue and violet end of 

 the spectrum. 



Of green filters there is a very wide selection. The most 

 useful is what is known as Zettnow's filter, in which a satu- 

 rated solution of copper nitrate is mixed with chromic acid in 

 water. This filter completely cuts out both the blue-violet and 

 the red end of the spectrum. The proportions of the two 

 solutions may be varied so that a greater or less absorption of 

 either end may be obtained at will. The most satisfactory way 

 is to adjust the solution by means of a hand spectroscope until 

 the part of the spectrum not required is eliminated. 



Another convenient way of making green and yellow-green 

 filters is to keep at hand a saturated solution of acetate of 

 copper and bichromate of potash ; both the solutions should 

 be strongly acidulated with glacial acetic acid. They may be 

 combined in any suitable proportion, and will give a strong 

 visual green or yellow-green filter, the predominance of either 

 colour depending on whether there is an excess of the green 

 solution (acetate of copper), or of the orange solution (bichromate 

 of potash). If on combining these two solutions any precipitate 

 is thrown down, it shows that they are not sufficiently acid, and 

 more acetic acid should be added to re-dissolve the precipitate. 

 Fig. 67 e is a photograph of the absorption-spectrum of these 

 solutions combined to give the greatest absorption at the violet 

 end of the spectrum, while transmitting as small a proportion of 

 the red as possible. 



Of green aniline dyes, malachite-green, methyl -green, naph- 

 thol-green, acid-green, and iodine-green will be found useful. 

 Naphthol-green is visually somewhat yellow-green or emerald, 

 whereas the other four are blue-green. Kecently a new dye 

 known as rapid-filter green has been introduced, which results 

 in well-marked absorption in the region of the spectrum between 

 wave-lengths 4500 and 5700. It also has the advantage that 

 its general absorption is not so great as in some other green 



