Dr. Forster on the dispersive Power of the Atmosphere. 193 



atmosphere to great variations in this its power of reflecting, 

 refracting and dispersing the rays of celestial luminous bodies, 

 and consequently the necessity of some further corrective 

 tables of refraction ; and to show what particular modifica- 

 tions of cloud produce these properties of the atmosphere in 

 the highest degree by being diffused therein. 



Previous to entering into the ensuing inquiry, I wish to 

 caution the general reader against ever confounding reflection, 

 refraction, and prismatic dispersion, which are distinct pro- 

 perties. For example, the lightness of the sky in the day time 

 is produced by the reflective power ; while its appearing of a 

 blue colour is an effect of a certain property which accom- 

 panies it, whereby certain blue rays are separated from the 

 rest. As a more striking example of the dispersive power, I 

 may instance the beautiful red, crimson, and yellow colours of 

 the clouds produced on certain occasions by the light of the 

 sun refected by their surfaces, being refracted and dispersed by 

 the aqueous atmosphere in its passage to the earth's surface. 



I am aware that the clouds themselves may on some occa- 

 sions possess the property of separating light ; and to this cause 

 I was formerly inclined to ascribe their colour; but subse- 

 quent experience and reflection have convinced me that the 

 most brilliant tints seen in the clouds are the result of the 

 dispersive power of the aqueous vapour existing in a state of 

 general diffusion in the lower atmosphere, whereby the light 

 reflected from the surface of the cloud is prismatically dispersed 

 into separate rays in its passage through the lower atmosphere 

 to the earth. 



It must be borne in mind, that my observations below do 

 not relate to those remarkable and special refractions which 

 occur in certain definable clouds, as the rainbow, which re- 

 sults from reflections and special refractions of certain rays at 

 a determinate angle in the nimbus or raincloud, producing con- 

 centric rings of colours conformable to their different degrees 

 of refrangibility. Nor do I include those special refractions 

 of light in its passage through the cirrostratus, called halos, 

 whereby concentric coloured rings are produced in the order 

 of their respective refrangibility, and reflected at an angle 

 which equals the angle subtended by the semidiameter of the 

 halo. 



I allude in this paper to the ordinary effects of more thinly 

 diffused and almost invisible cloud in the atmosphere, through 

 which the luminous celestial bodies appear clear and distinct, 

 but which nevertheless refracts and disperses their rays in some 

 degree, even in apparently the clearest nights. 



Refraction has been hitherto considered as so general an 

 effect of the atmosphere, and so unconnected with any par- 



Vol. 63. No. 31 1. March 1824.. B b ticular 



