PHYSICS. 611 



iilar to the fixed mirror and the movable mirror so arranged that its axis 

 of rotation coincides with one of its edges and cuts the optical axis. The 

 secondary mirror, instead of being fixed, is movable about an axis par- 

 allel to the plane of the graduation and perpendicular to the ox>tic axis. 

 Under these conditions the image twice reflected from any given point 

 will be displaced in a plane perpendicular to that of the circle when 

 the secondary mirror turns about its axis. If the telescope be sup- 

 pressed and a simple vertical slit be placed between the eye and the axis 

 of rotation of the movable mirror, and if the mirror be silvered all over, 

 an object can be seen above or below it, and it can be made to coincide 

 with the image of another object twice reflected and thus their angular 

 distance determined, projected on the horizon. {C B., April, 1885, c, 

 1120; Phil. Mag., June, 1885, V, xix, 516.) 



Kriiss has investigated the influence of temperature on the refraction 

 of prisms in order to determine its comparative importance in spectrum 

 measurements. His measurements were made by means of a microm- 

 eter which moved the observing telescope in connection with an eye- 

 piece micrometer carried by it. Three prisms were measured at dif- 

 ferent temperatures. From the results it appears that all spectrum 

 lines shift by rise of temperature. In glass prisms this shifting is to- 

 ward the violet ; in quartz prisms toward the red. In the triple glass 

 prism a change of temperature of about 5° suffices to alter the measure- 

 ment of wave-length by an amount equal to the distance between the 

 D lines. Moreover this shifting increases with the refrangibility of the 

 region examined. {Ber. Berl. Cliem. Ges., December, 1884, xvii, 2732 ; 

 Am. J. 8ci., March, 1885, III, xxix, 251.) 



Christiansen's original experiment consists in immersing glass powder 

 in a mixture of carbon disulphide and benzene in such proportions that 

 for one part of the spectrum the indices of the solid and of the fluid are 

 the same. ( Wied. Ann., xxm, 298.) Eayleigh has improved this experi- 

 ment by using aflat-sided bottle to contain the preparation, and by hav- 

 ing the same kind of glass for the bottle and for the powder. The sides 

 of the bottle are worked flat, like plate glass. Ordinary flint does not 

 seem to work as well as plate glass, though optical flint answers well. It 

 is more important that the powder should be homogeneous in itself than 

 that it should correspond very accurately with the glass of the bottle. 

 Properly arranged rays lying within a very narrow range of refrangi- 

 bility traverse the mixture freely, but the neighboring rays are scattered 

 laterally much as in jyassing ground glass. Two complementary colors 

 are therefore exhibited, one by direct the other by oblique light. In 

 order to see these to advantage there should not be too much diflused 

 illumination, otherwise the directly transmitted monochromatic .light is 

 liable to be greatly diluted. {Phil. Mag., October, 1885, V, xx, 358.) 



Some years ago Eayleigh examined the effect upon definition of small 

 disturbances in the wave surfaces from their proper forms, and con- 

 cluded that the aberration of a plaijo-convex lens focusing parallel rays 



