360 RADIANT HEAT.% 



metrical observations; while the observations accord much better with 

 the law of Fresnel than with that deduced by Mr. Potter. In the 

 instance of metals Professor Forbes considers Mr. Potter's discover}' 

 verified, that the reflection is less intense at higher angles of inci- 

 dence; he has not yet been able to verify Professor Maccullagh's 

 inference, that it has a minimum before reaching 90°; and, lastly, he 

 observes that the quantity of heat reflected irom metals is so much 

 greater than Mr. Potter's estimate for light as to lead him to suspect 

 that all that gentleman's photometric ratios are too small; this would, 

 nearly account for their deviations from Fresnel' s law. He has also 

 made some attempts for verifying that law by observations on heat 

 polarized in opposite planes. 



Mr. Potter, it is well known, mainly founds his objections to the 

 undulatory theory on the discrepancy between Fresnel' s law for the 

 intensities of reflected light and his own photometrical determinations. 

 He has therefore naturally been led into some controversial remarks 

 on Professor Forbes' s results in a paper in the London and Edinburgh 

 Journal of Science, to which Professor Forbes has replied. 



Considering that the whole inquiry is as yet confessedly in an 

 incomplete state, any further observations upon it in this place would 

 be premature. 



Conclusion. 



In thus reviewing the different points of inquiry which have been 

 of late pursued relative to radiant heat, and the several important 

 discoveries with which that research has been rewarded, I have for 

 the most part preserved, under each head, the chronological order. 



The progress of discovery is here, I trust, too clearly marked to 

 allow any real ground for these questions as to priority and originality, 

 which have given rise to so much unhappy controversy between rival 

 philosophers, or to the less open but equally lamentable manifesta- 

 tions of jealousy, in ambiguous expressions of claims, into which men 

 of science have been sometimes betrayed. The dispassionate reviewer 

 of the history of discovery at once best avoids all such controversial 

 topics, and fulfils the demands of critical justice, by a simple but 

 careful statement of facts. 



In the present instance it appears to me that the share of credit 

 due to the distinguished parties, respectively, who have co-operated 

 to introduce the discoveries above reported is sufficiently well-marked, 

 and certainly ample enough in each instance to confer the highest 

 celebrity on those Avho have borne the chief portion of the labor. 



To the continental philosophers belongs the first invention of the 

 instrument, without whose aid none of these investigations could have 

 been accomplished; while all the earliest and most important discove- 

 ries of the varying diathermancy of substances; the knowledge of the 

 singular constitution of rock-salt, (which has placed a new instrument 

 in the hands of the experimenter;) and the capital fact, disclosed by 

 means of it — the refraction of heat from dark sources; together with 

 the very singular phenomena of the changes in the nature of heat by 

 transmission through certain substances; the remarkable effect of 

 smoked rock-salt; the circularly polarizing power of quartz for heat — 



