246 SEC. 7. LIGHT. 



970. Apparatus for illustrating the Colours of double 

 refracting Bodies, in the form of a Gothic window, composed of 

 gypsum plates systematically arranged according to the colours ; 

 with a blackened glass plate belonging to it. 



Prof. Dr. J. J. Oppel, Franhfoi't-on- Maine. 



971. Some characteristic Drawings for illustrating the 

 Stroboscopical Principle, with manifold movements, as, for- 

 wards and backwards, centripetal and centrifugal, undulatory, 

 oscillatory, and quite irregular. 



Prof. Dr. J. J. Oppel, Frankfort- on- Maine. 



Is a collection of those principal forms of periodic movements which . can 

 be represented stroboscopically. 



IX. MISCELLANEOUS. 



897. First Heliostat, invented by 'sGravesande. 



Prof. Dr. P. L. Rijke, Ley den. 



(See 'sGravesande's "Physices Elementa Mathematica, " ed. III., Tom. 

 I., page 715.) 'sGravesande was an eminent Dutch geometrician, b. 1688, 

 d. 1742. 



898. Heliostat, G. Johnstone Stoney's modification, made by 

 Spencer and Sons, Dublin, a cheap and useful form. 



Prof. W. F. Barrett. 



971aa. Self-adjusting Heliostat, mounted on a declination 



bar magnet. Dr. Stone. 



97 lab. Mounted Prism of Quartz. Dr. Stone. 



97 lac. Mounted Prism of Iceland Spar. Dr. Stone. 



971ad. Equatorial Heliostat for maintaining a reflected 

 beam of solar light in a constant position, applicable to places 

 whose aspect will admit of a beam of light from the north only 

 being obtained. 



By changing the dial wheel of the clock, it may be used from 

 the south as a polar instrument. Conrad W. Cooke. 



It consists of a plane mirror, adjustable in declination, revolving upon a 

 polar axis and driven by a clock. The clock has two interchangeable dial 

 wheels whose circumferences are respectively one-half and one-fourth that of 

 the wheel upon the polar axis of the mirror. By using the smaller, the mirror 

 is driven round once in 48 hours, and if the instrument be placed due north 

 of the spot to which the reflected beam is to be directed, and in the same 

 equatorial plane with it, and adjusted in declination, the beam will be main- 

 tained in that plane and in a fixed meridional direction. 



By substituting the larger dial, the mirror will rotate once in 24 hours, and 

 if the instrument be placed due south and in such a position that its polar 

 axis passes through the spot to be illuminated, the beam will be equally constant 

 in direction from the south. 



