Dr. J. Reade 0)i the Nature of Light and Shadow. 109 



the sphere contained by <r and the two meridians. This pro- 

 position is rigorously true, and is only a particular case of a 

 more general theorem demonstrated by Professor Gauss in the 

 Memoirs of the Royal Society of Gottingen : but it must be 

 observed that the trilateral figure on the surface of the sphere is 

 not a spherical triangle, because the line <r is not contained in 

 the plane of any great circle. And as the line o- depends upon 

 the geodetical line s, which, supposing the latitudes and the dif- 

 ference of longitude to remain unchanged, varies with the ex- 

 centricity, it follows necessarily that the sum of the azimuths 

 at the extremities of 5, is not independent of the excentricity, 

 but varies from one spheroid to another. The sum of the 

 azimuths mentioned is not, in any spheroid, exactly equal to 

 the sum of the angles at the base of a triangle on the surface 

 of the sphere formed by the two meridians, and a great cii'cle 

 which cuts them at the latitudes of the stations. 



Jan. 13, 1829. J. IvORY. 



XVII. On the NatU7-e of Light and Shadoiio, demonstrating that 

 a Black Shadow can be rarefied, imthout Refraction, into all 

 the Colours of the Rainbow. By Joseph Readi:, M.D. 



To the Editors of the Philosophical Magaziiie and Annals. 

 Gentlemen, 

 T BEG leave to return you my thanks for the correctness 

 -*■ with which you have published my experiments on light*, 

 and I hope the following novel experiment may be favourably 

 received by your scientific readers. 



Experiment 1. — Having placed a table at about ten feet 

 from a well lighted window, I placed on it a candle in a high 

 candlestick. 1 now held a quire of white paper parallel to the 

 table, and at right angles with the lighted candle : on holding 

 this paper rather close to the blaze, two shadows were pro- 

 duced by means of a piece of coiled paper held immediately 

 near the quire ; the one next to the candle was a bright orange, 

 the other a bright blue. On turning the quire of paper to- 

 wards the window, so as to cut off' the light of the candle, this 

 orange shadow changed to a perfect black ; and on turning 

 the (juire of paper towards tlie candle, and excluding the light 

 of the sun, the blue shadow likewise changed to a perfect black. 

 Here I changed orange and blue colours into black, and vice 

 versa, witiiout any possibility of refraction. This experiment 

 may be nuide by holding the paper behind the candle. 



Experiment 2. — The former experiment was made with the 

 paper between the candle and the window : 1 now held the 

 paper close behind the candle, and perceived two shadows, the 

 * Sec Phil. M;if;. vol. Ixiii. |). ','7, Ac. 



one 



