[ ^01 ] 



LXX. Historical Mem or midn respecting Experirvenfs inleyuJed 

 to ascertain tlie calorific Powers of the different prismatic 

 Eaijs. By T. S. Evans, LL.D.F.L.S. 



To Mr, Ti/loch. 



Sir, — i_yiscov£RiJiS in science have heen generally made by im- 

 perceptible "steps ; and improvements in tlie arts have succeeded 

 each other 'hy such small gradations, that in many cases the shade 

 of difference has scarcely been apj)arent. Inventions have often 

 been brought before the public with so little alteration from 

 others which they possessed before, that the claim of novelty 

 could scarcely be allowed them ; and advances in tlie state of our 

 intellectual knowledge have sometimes been made by such simple 

 and evident means, that it has been rather a subject of astonish- 

 ment they were not known long before. Instances are not 

 uncommon in the liistorv of science, of two persons at the 

 same time making the same discovery or improvement, at a 

 distance from each other, and without ever having had the least 

 commimication: but many more are to be met with, where one 

 person has succeeded another in the fame invention, and both 

 have been equally entitled to tiie palm of merit, 



The.".e reflections have arisen from considering our present 

 knowledge of the properties of light, particularly that of the dif- 

 ferent degrees of heat communicated to the tiiermometer by the 

 coloured rays arising from their decomposition by means of the 

 prism ; but they would pro!)ably have arisen from considering 

 the history and progress of any otlicr department cf science or 

 art, as thev have all proceeded neailv in a similar way. 



That light could be refiected and' refracted, was well known 

 to the ancients, as a])pears from the few remains that are left 

 of their writings. Contrary to the opinion that has lately been 

 generally entertained, the celebiated work of Ptolemy upon Optics 

 is still in existence. A copy of it is yet in the Imperial library 

 at Paris ; and some extracts from it have been lately given by 

 the Chevalier Delambre, i)i the Connoissrmce des Temps for the 

 year 1814, by which, and the treatise of Euclid on Optics, it 

 appears that these two properties were well understood in their 

 {jmes by the Greeks and Egyptians. 



Long before it was certainly known that light proceeded by a 

 progressis'e motion. Dr. Hook had asserted that this motion 

 could not l)y any means be instantaneous; yet it may fairly be 

 presumed that, up to the time of this discovery, mankind had no 

 idea of any thing in nature moved with such an incredible 

 velocity ; and accordingly v/e find Galileo and the members of 

 the Academia del Cimento endeavouring to discover what dif- 



Vol, 45. No. 206. June 1315. C c ference 



