C. GENERAL PHYSICS. 155 



and the forms of the prominences quite as well as with the 

 instruments ordinarily applied to that class of observations. 

 He finds the diflVaction gratings much easier and lighter 

 manao-ed than the train of prisms, and anticipates that for a 

 certain class of observations they will supersede them. 

 4 i>, 1873, 473. 



THE MECHANICAL COMBINATION OF COLORS. 



Mr.F. J.Smith has designed a very convenient and elegant 

 instrument to illustrate the phenomena of combination of 

 colors. The instrument is designed to show the color that 

 results from a mixture of all or any of the colors of the spec- 

 trum given by .any. It consists essentially in a disk which 

 can be caused to revolve very rapidly ; at the centre of the 

 disk is fixed a small plain mirror at an angle of forty-five de- 

 crees to the axis of revolution. In front of the mirror is 

 placed a prism, while in front of the prism, and like it at- 

 tached to the disk, there are placed slides, which can be 

 changed so as to cut ofi* any part of the rays of light. A ray 

 of lio-ht is admitted throuo-h a small slit attached to the mir- 

 ror, in such a way that the light, moving along the axis of 

 revolution, passes first of all through the slit, then impinges 

 upon the mirror, and is reflected at right angles to the axis ; 

 then, passing through the prism and the slides, it falls upon a 

 fixed screen which entirely surrounds the movable disk; 

 the disk being set in revolution, a coloi-ed band falls upon the 

 screen, and moves around it as the disk revolves. When a 

 certain velocity is arrived at, the colors combine, so that, look- 

 ing upon the screen, we see the light Avhich originally entered 

 at the slit, and of the same color, unless any portions of this 

 liglit are cut ofi^ by the slides when the revolving color is 

 seen upon the screen. 12 A, 1873, 262. 



THE SOLAR HEAT REFLECTED FROM LAKE GENEVA. 



Dufour has, by means of hollow, blackened bulbs with a 

 central thermometer, investigated the comparative amount 

 of heat received from the direct rays of the sun, and that re- 

 ceived from those rays that are reflected from the surface of 

 the water of Lake Geneva. Simultaneous observation was 

 also made of the atmospheric temperature, and the whole 

 series of observations were repeated at five difl*erent stations. 



