226 SEC. 7. LIGHT. 



The apparatus serves to perform the radiometer experiments, described 

 by the exhibitor in Carl's "Kepert. der Experimental Phys., 1876, Heft. 2." 

 Compare also the description annexed to the apparatus. 



900. New Radiometers. Dr. H. Geissler, Bonn. 



901. Radiometer. John Browning. 



These instruments are set in motion by either light or heat ; they consist of 

 four small discs on two arms at right angles to each other ; the discs may be 

 of pith or mica ; those exhibited are made of mica, as the} r appear to be the 

 most sensitive to minute traces of light. One side of each disc has a dead 

 black surface. The action of light or heat repels the black surfaces, and 

 continuous motion is obtained so long as any light or heat faDs on them. 



902. The late Prof. T. T. Miiller's Apparatus for 



illustrating the influence of the intensity of light on its rapidity 

 of propagation (Poggendorff's Annalen, 1872, cxlv. p. 86.) 



Prof. A. Mottsson, Zurich. 



Use is made of Newton's rings, produced between a plane glass a, and 

 another glass b (the latter very slightly convex), which may be separated 

 in a known manner so as to produce differences of progression up to 

 50,000 waves. At this distance, the convex glass b, on which a rested at 

 first, radiates in the centre of a square iron vessel c on mercury. The three 

 screws of the support d which surrounds the glasses are fixed apart. Then, 

 the mercury having been allowed to flow out (through the cock 6), c is 

 brought down and fixed also, by means of three other screws y. The 

 distance can be calculated with great precision by means of the weight of 

 the mercury, and the known area of the vessel c. 



The luminous point used is the small image at the opening of a collimatiug 

 tube e, lighted by a monochromatic sodium flame, upon the hypothenuse 

 surface of a small prism y*. From this point the rays diverge and fall on 

 the lens g, placed on the glass c, which makes them parallel. These rays 

 return, with interference, from the two reflecting faces, towards the pointy, 

 where the eye is placed close to the prism. 



Now, if the intensity of the light be lessened by interposing absorbing 

 glasses, it will be seen that the greater the difference in the number of waves 

 the more the lines change place, the increase of rapidity being proportionate 

 to that of intensity. 



V. REFLECTION, REFRACTION, AND DIFFRACTION. 



903. Total Reflection Apparatus, for the projection of 

 objects placed in a horizontal position. 



J. and A. Molteni, Paris. 



904. Small Prism for double reflection. Laurent. 



905. Coloured Rings on an 80 millimetre tripod. 



Laurent. 



905 a. Fresnel's Parallelepiped. M. Lutz, Paris. 



