POLARISATION OF THE ATMOSPHERE. 249 



general conclusion " that the atmospheric polarisation is subject to a diminution 

 during the morning, and to an increase during the evening, without one's being 

 able to assign with certainty the precise hour of the minimum polarisation." 

 These changes Dr Rubenson found to be often influenced by perturbations com- 

 monly of short duration, and taking place indifferently at all hours of the day. 

 They frequently arise from clouds or smoke, and probably often from cirrus too 

 faint to be seen. According to Dr Rubenson, the blue colour of the sky, in a 

 normal state of the atmosphere, and 90° from the sun, is feeble at sunrise, in- 

 creases rapidly in intensity, and attains to its maximum some hours before noon, 

 the number of hours being different at different seasons. The intensity of the 

 colour diminishes towards noon. It then increases, reaches a second maximum 

 after some hours, and then diminishes quickly towards sunset. The relation be- 

 tween the blue colour of the sky and the intensity of its polarisation, is a pro- 

 blem which remains to be solved. 



In 1859, M. Liais made observations on the polarisation of the atmosphere 

 during his voyage from France to Brazil, and at San Domingo in the bay of Rio 

 Janeiro. His observations were made at the beginning of dawn and at the end 

 of twilight, with the view of determining the height of the atmosphere. From 

 the observations made at sea he obtained 320, and from the observations made 

 on land 340 kilometers, or 212 miles, as the height of the atmosphere* 



The most recent observations on the polarisation of the atmosphere were 

 made by M. Andres Poey, between 1862 and 1864, under the tropical sky of the 

 Havannah. The observations themselves have not been published ; but he states, 

 as one of the most important of their results, that " at sunrise and sunset the 

 system of atmospherical polarisation ought necessarily to present two planes of 

 rectangular polarisation, one vertical, passing through the eye of the observer 

 and the sun, and the other horizontal, with four inversions of the signs, and four 

 neutral points 90° from each other." 



M. Poey adopts my theory of atmospherical polarisation, and the analogy 

 which I pointed out between the lines of equal polarisation and the isochromatic 

 lines of biaxal crystals, and between the same lines and those of uniaxal crystals 

 when the sun is in the zenith, — the neutral points now meeting in the sun.f 



It will be seen, from the preceding details, that the subject of atmospherical 

 polarisation has become one of the most important branches of optical meteor- 

 ology. It has already thrown much light on the constitution of the atmosphere ; 

 and when it has been studied in different climates, and at different altitudes 

 above the sea, by Alpine travellers and scientific aeronauts, it will doubtless have 

 still more valuable applications. 



* Comptes Itendus, &c. torn, xlviii. pp. 109-112 



f See Comptes R-ndus, &c. tom.'lx. p. 781, Avril 17, 1865. 



