1818.] perfectly Achromatic. Ill 



must be cemented to the surface A B. By this means we shall 

 have a prism in which the imperfections of the polishing are 

 removed, while the two images are perfectly white, and nearly 

 equally luminous. If we wish one of the two images to be still 

 more perfect, it must be done at the expense of the other image, 

 by using a cement of the same refractive power as that which, 

 corresponds to the image which we wish to improve. This 

 compound prism will admit of still farther improvement, by 

 cementing, in every case, the prism of glass C e d to the calcareous 

 spar with a cement of the same refractive power with that which 

 corresponds to the ray r m, and the other prism B e d D, with si 

 cement whose refractive power is equal to that which cor- 

 responds to the ray r n. 



These practical directions respecting the nature of the cement 

 are of greater importance than we are apt to imagine. They are 

 deduced from a series of new experiments on the action of 

 crystallized surfaces, and on the extension of the doubly refract- 

 ing force beyond the surfaces of bodies, which will soon be 

 published. 



Fenlaw, Oct. 15, 1817. 



Article V. 



Of Isothermal Lines, and the Distribution of Heat over the 

 Globe. By Alex, de Humboldt.* 



The unequal distribution of heat over the globe is one of those 

 phenomena which has been long known as a general fact, but 

 which cannot be exactly ascertained, with respect to its particular 

 laws, until we have more correct data furnished by observation 

 and experiment. To furnish these data is the immediate object 

 of this paper ; they are deduced from a great number of facts 

 which have not been published ; and if they are not sufficient to 

 enable us to form a correct theory, they may, at least, lay a foun- 

 dation for it, and will be useful in pointing out to travellers those 

 objects to which they ought especially to direct their attention. 



The distribution of vegetables and of organized beings in general 

 depends upon the circumstances that are connected with latitude, 

 longitude, and elevation ; and of these one of the most important 

 is atmospherical temperature. The means which the author 

 enjoyed of making observations, during his residence in South 

 America, have enabled him to establish some very valuable data, 

 which could not have been obtained in any other situation, more 

 especially those which require to be made at great heights above 



• Abridged from the third volume of thcMcmoira of the liociety of Arcueil. 



Vol. XI. K° III. M 



