On Refraction. 



It was not till after the year 1672, that a 

 tolerably near table of refraction made its ap- 

 pearance, when the elder Cassini took the 

 subject into consideration*. What led to 

 this was the voyage of Richer to Cayenne in 

 that year, upon the utility of which some 

 very excellent remarks were made by Cassini, 

 showing how far observations made in a situ- 

 ation so near the equator tended to confirm 

 or disprove certain theories derived from 

 observations made in Europef. Several very 

 useful deductions were drawn from a com- 

 parison of those made both at Paris and Cay- 

 enne; among others the refraction was set- 

 tled upon more accurate elements than here- 

 tofore |, and a new Table computed, for the 

 first time, of its quantity for all degrees, up 

 to the zenith ; an abridgement of which is 

 given in the margin. 



From the relation of his grandson, it ap- 

 pears, however, that Cassini had at one time 

 computed three tables of refraction for all 

 altitudes : one for winter, another for summer, 

 and a third for spring and autumn : but several 

 doubts having been suggested to him re- 

 specting this arrangement, although in ap- 

 pearance conformable to nature, and princi- 

 pally the observations of Richer at Cayenne, 

 where the refraction was found little dif- 

 ferent from that at Paris, he changed his 

 opinion ; and, judging that since the great 

 difference of heat of the torrid zone, from 

 that of the temperate, which we inhabit, 

 does not cause sensible differences in the re- 

 fraction ; therefore, the greatest heat or cold 

 of our climate could not change it much ; 

 and he then fixed upon one table, which was 

 that used by the astronomers of the Royal 

 Observatory of Paris, up to the vear 1745 §. 



It was alwavs thought, before the time 

 that the refraction did not extend its 



343 



Table continued. 



of Cassini, 

 influence higher 



* Mem. de 1* Acad, avant son renouv. torn. v. p. 81. 



+ In the observations of Picard made in various parts of France, in the 

 year 1674, there are several for ascertaining the retraction; and a table is 

 given from them for each degree of altitude up to 22°. — Mem. deVAcud. av. 

 fonreimuv. torn. iv. p. 111. 



X Mem. del" Acad, avant son renouv; torn. v. p. 105. § Ibid. 1745. 



Y 4 than 



