132 ANNUAL OF SCIENTIFIC DISCOVERY. 



that water is not so colorless as might lie supposed. A tin tube, fif- 

 teen inches long and three inches in diameter, was placed horizon- 

 tally on a stand, and half filled with water. The tube was closed by 

 plate-glass at each end, and a beam of "electric light thrown through 

 it. By this means an image of the contents of the tube was projected 

 on a white screen. That portion of the tube which was filled with 

 air allowed the rays to pass through unchanged in color, when they 

 formed a white semi-circle on the screen ; but the rays which passed 

 through the stratum of water were seen to have had a greenish blue 

 color communicated to them. The color was found to vary from a 

 pure green up to a blue, according to the purity of the water. It is 

 thus evident that the color of water is very appreciable ; for, in a 

 thickness of only fifteen feet, it exhibits a very considerable amount. 



Dr. G. C. Wittstein, in a paper read before the Bavarian Academy 

 on the color of water as found in 'nature, sums up as follows : 



Pure water is not colorless, but of a blue tint, which is not altered 

 by the mineral ingredients present. Any deviation from the blue 

 color is caused by organic substances, of the class termed uhnic acids, 

 held in solution by alkalies. In proportion to the quantities of these 

 organic salts the color reaches from a bluish green to yellow and 

 brown. The amount of alkali determines of course the solubility of 

 ulmin and the shade of color, and is itself dependent on the nature 

 of the bed or rock with which the water conies in contact. As a 

 general rule it may be assumed that running water is the softer the 

 nearer its color approaches brown, and the harder as it becomes 

 blue. 



SUBAQUEOUS EXPLORATIONS. 



A lorgnette for subaqueous exploration has been prepared by Dr. 

 C. M. Cresson, of Philadelphia. Its peculiarity consists in placing a 

 Nicol's prism of Iceland spar between the object-glass and the eye- 

 piece, which removes the greater part of the bright light reflected 

 from the surface of the water, and thus renders objects beneath wa- 

 ter more distinctly visible. 



THE THREE PRIMARY COLORS. 



The following is an abstract of a lecture on " The Theory of the 

 three primary Colors," delivered by Prof. J. Clark Maxwell, of 

 King's College, London : 



Colors may be considered in their relation to the arts, to optics, 

 and to physiology. By mixing various pigments the artist is enabled 

 to imitate the colors of nature, and to that object his study of colors 

 is limited. The researches of the optician and physiologist have a 

 far wider field. The foundation of the theory of the composition of 

 colors was laid bare by Newton ; he showed that every color in na- 

 ture is procured by the mixture in various proportions of the differ- 

 ent kinds of light into which white light is divided by refraction. By 

 means of a prism he obtained a spectrum, consisting of seven colors 

 red, orange, yellow, green, blue, indigo, violet ; of these, red, blue, 

 and green have been termed primary. By means of a prism we can 



