PEALE] ON THE COLOR OF WATER. 379 



Herr Boas, of Kiel, experimented with a ziuc tube 4G feet long, the 

 ends being closed with glass plates. Distilled water gave a fine deep 

 blue-green color; the red end was absent, and the yellow feeble, and the 

 maximum brightness in the green. Water of the Kiel supply let no 

 light through the length stated (46 feet); with half the length it ap- 

 peared deep orange, blue and green failing entirely. He also performed 

 quantitative experiments. 



In his quantitative experiments the author illuminated two screens from the same 

 light-source (sodium light or a gas flame), before which was placed red glass, or sul- 

 phate of copper solution. The light from one screen went through water in a tube; 

 that from the other along the tube outside. 



Both beams were brought into a position for comparison by means of 

 total reflection prisms; the screens were shifted till equal brightness was 

 reached, and from their position the coefficients of absorption could be 

 approximately inferred. The decrease of absorption towards the blue 

 in the case of distilled water is thus clearly shown.* 



He also found that the light issuing from the water was weakly po- 

 larized in a plane passing through the sun and the direction of the 

 beam. As to fluorescence his experiments had a negative result. 



Soret, in a letter to Professor Tyndall, from Geneva, March, 1869, 

 maintains that the blue color of Lake Geneva is due to the suspended 

 solid particles, from the fact that he established by direct experiment 

 that this light presents phenomena of polarization identical with those 

 of the light of the sky.t His experiments were confirmed by experi- 

 ments on the water of Lake Lucerne made by Prof. E. Hageubach. 



In a very interesting article in the Philosophical Magazine | Mr. E. L. 

 Nichols argues that the effect of the light on the nerve termini in the 

 eye must be taken into account. These are variously aflected by the 

 different colors and have their own laws. For feeble rays violet nerves 

 are very sensitive, and as the intensity increases these become tired or 

 dazzled and are incapable of receiving their full share of effect. For 

 rays of dazzling intensity the red nerves are in the most sensitive con- 

 dition, while those affected by the more refrangible components of the 

 ray are comparatively dull. Of simple colors, as the brightness of the 

 ray increases, red and green change to yellow, and blue becomes white. 

 Passing cloud-shadows show that what under shadow is indigo blue 

 in direct sunlight becomes silver gray. He says the difference between 

 moonlight and sunlight is only in intensity, but the yellow impression 

 of sunlight giv^es place to a bluish one. A ray that is white in ordinary 

 light becomes blue as it gets faint. Eays less refrangible than blue are 

 absorbed in the passage of light. 



In our experiments detailed in the first part of this chapter we noticed 

 that when a bright ray of sunlight was passed through the tube in the 

 case of distilled water the greenish light acquired a yellowish tiuge. 

 The blue light of Cleopatra water became colorless or white, and the 

 column of air when in shadow acquired a bluish tinge. On one occasion 

 night came on while we were examining a specimen of Schuylkill Eiver 

 water, and as the light faded the beam transmitted through the tube 

 became orange and finally quite ruddy. This is in accordance with the 

 facts just presented. 



*Nature, 1881. 



t Philosophical Magazine, 4th scries, vol. 57, p. 34.5, May, 1869; Compt. Rendus, 

 tome 68, p. 911, April, 1869; Archives des Sci. Phys. etNat., tome 35, p. 64, May, 1869, 



tANew Explanation of the Color of the Sky, by E. L. Nichols; Philosoph. Mag., 

 5th aeries, vol.8, No. 51, p. 42^43'3, 1879. 



