45 



which, as before, is incident at an angle of 18° on the plate. In this 

 position, the light falling perpendicularly on the narrow face, passes 

 through it unbroken, and being within the glass reflected from the 

 rulings, the coloured pictures of all the twelve systems are now ex- 

 hibited in the microscope. If the colours which these systems now 

 show, and which have been formed in the glass^ be compared with 

 the former colours m the air, the system F, which is deep red, har- 

 monizes with the system A in the air spectrum, and the systems G, 

 H, I, K, L and M in the glass spectrum, according to their order 

 and colour, correspond to the systems B, C, D, E, F and G in the 

 air spectrum ; and the author considers that the comparison of the 

 foregoing values in the systems — 



A and F, E and K, 



B and G, F and L, 



C and H, G and M 



D and I, 



Fhows that the lengths of undulation for the same colour in the air 

 and in the glass are in the ratio 1*53 to 1, which is exactly the index 

 of refraction of this glass. He further remarks, that, as the same 

 period of undulation belongs to the same colour, the velocities of 

 propagation in air and in the glass must be in the ratio of the mean 

 value of the distances of the lines in the first seven systems. A, B, C, 

 D, E, F, G, to the mean value of the distances of the lines in the 

 last seven systems, F, G, H, I, K, L, M, or as 1*53 to 1 ; and that 

 both results agree perfectly with the deductions from the wave- 

 theory of light. In conclusion, the author points out the extraordi- 

 nary degree of accuracy required in drawing these lines. It is stated 

 that if inequalities amounting only to '000002 line occur in the sy- 

 stems F, G, H, I, K, L, M, stripes of another colour will appear in 

 them ; and if the distance of the lines in M be diminished by that 

 quantity, all colour disappears. 



The following communication was also read : — 



Extract of a letter from Professor Kamtz to Lieut. Colonel Sabine, 

 on " Corrections of the Constants in the general theory of Terrestrial 

 Magnetism." Received April 3, 1851. 



Translation. 



Dorpat, ^ January 1851. 



From the active zeal with which you pursue the phenomena of 

 terrestrial magnetism, and collect all the facts which can conduce to 

 the elucidation of this difficult subject, I think that some researches 

 with which I have occupied myself will not be wholly uninteresting 

 to you ; and I therefore address you the following lines, which I have 

 also permitted myself to write in ray own language. 



Some years ago I employed myself in endeavouring to correct 

 the constants which Gauss has given for the earth's magnetism. 

 The process I adopted was by considering the horizontal and verti- 

 cal components separately ; but when I learned that Erman had the 



