COLOR FILTERS 1 31 



amount of curvature six hours later. The test will have greater 

 value if two chambers are provided and the tests are made simul- 

 taneously, with lots of seedlings that have stood in a dark room 

 for 24 hours. Examine the solutions by means of the spectro- 

 scope and ascertain what rays are absorbed and what are trans- 

 mitted by each. This will lead to conclusions as to the part of 

 the spectrum most active in inducing phototropic reactions. 



200. Color Filters. 1 Red. .05 g. water free cantharides green 

 crystal violet dissolved in sufficient alcohol. Dilute^-to 1 L. with 

 distilled water. In a layer of 20 mm. thickness a red and wide 

 blue violet band is given. The latter may be removed by a solu- 

 tion of potassium chromate 10 to 100 water in a layer 20 mm. in 

 thickness. 



Yellow. Dissolve 30 g. of crystals of nickel sulphate in 100 

 cc. of distilled water. Used in a layer 20 mm. thick it absorbs 

 red only. Now pass the light through a solution of potassium 

 chromate 10 g. in 100 cc. of water in a layer 15 mm. thick. 

 This will absorb the blue. Then pass the remaining light through 

 a solution of potassium permanganate .025 g. in 100 cc. of water. 

 Only orange yellow and a trace of red will remain. 



Green. Dissolve 60 g. crystals of copper sulphate in 100 cc. 



, of water and use in a layer 20 mm. thick. Only green and blue 



pass through it. The latter may be taken away by the potassium 



chromate solution described above. A wide green band with a 



trace of red remains. 



Blue, bright. Dissolve .02 g. of methyl green (Doppel-griin, 

 S. F.) which will give a bright bronze precipitate with chloride of 

 zinc, in 100 cc. of water. Used in a layer 20 mm. thick it allows 

 red and blue to pass. The red may be taken away by a solution 

 of 1 5 g. copper sulphate (crystals) in 100 cc. water in a layer 20 

 mm. thick. 



Blue, dark. Dissolve .005 g. crystal violet 5 BO in 100 cc. 

 Water and use in layer 20 mm. thick. Also 1 5 g. copper sul- 



1 Methods for obtaining pure colors used by II. Landolt in some work on "rota- 

 tions dispersion." Ber. Deut. Chem. Ges. 27 : 2872. 1894. 



