4iS' Remarks on the Enquiries of Dr. HerJchetrefpeEling Light and Hicit. 



The 17th, 18th, 19th, and 20th, are the ortly experiments which require any particular 

 notice, as feeming to countenance Dr. Herfchel's paradoxical opinions. In the firft, a 

 femicircular piece of pafteboard, covering the half of a large lens, received the coloured 

 prifmatic fpeculum, but permitted the " invifible rays" beyond the margin of the red to 

 pafs through the glafs. A heat of 45 degrees was caufed in the focus. But with what care 

 and nicety the experiment was performed maybe judged from the circumftance, that the 

 bulb of the thermometer appeared illumed by a reddifh tint. This perplexing occurrence, 

 however, only inflames the love of the marvellous. The Doftor very gravely proceeds to 

 enquire whether iifuiftbh rays can, by condenfation or accumulation, become vifible. He 

 nluft entertain indeed a lofty idea of the nature and value of experiment, thus to fet it in 

 oppofition to what arc accounted the didtatcs of common fenfe. Had the refuit proved 

 fuccefsful, what a triumph gained over the frailty of human reafon ! It would have taught 

 us to liften with humility to the dreams of a Platonift or vifionary of the prefent day, who 

 announces the new and fublime difcovery, that the addition of nothings m-aks^ fotnethiiig. 

 But Dr. Herfchel's furmife was not confirmed ; and on repeating his experiments with forae 

 little more attention, the effeft was only 21 degrees, being not the half of what was be- 

 fore produced. This, among other inftances, may fervc as a fample of the author's accu- 

 racy and circumfpeftion. But how did he difcover that pafteboard would intercept the 

 whole of the incident light ? He confidently regards this cover, applied too on the ve^ry 

 •furface of the burning glafs, as a perfeft diaphragm. Yet when an experiment, and the 

 only one in any degree conclufive, is adduced, tending to fupport an opinion mod repug- 

 nant to our general ideas, we might reafonably exped that every precaution would be ufed, 

 and the previous fteps at Jeaft fcrupuloufly determined. Common writing-paper, I have 

 found, tranfmits about one-half of the whole incident beam. What was the thicknefs or 

 the texture of the Do£lor's pafteboard, we are not informed. But even granting his expe- 

 rmients to be performed with accuracy, if the pafteboard permitted only the fixth part of 

 the light to pafs, this would have been fufhcient to produce the alledged effeft. And after 

 all, what 'reliance ought to be placed on obfervations which are at variance with every known 

 faft, and every eftabliftied principle .' 



The 19th experiment, whofe obje£t is " the refra£lion of invifible culinary heat," abfo- 

 liitely proves nothing. A hot cylinder of iron was placed near 3 inches from a lens above 

 an inch in diameter, and a thermometer at an equal diftance behind it in a pofition corre- 

 fponding to the fecondary focus. The thermometer rofe a degree or two as the iron cooled, 

 and diffufed its heat among the neighbouring bodies. Nor did it require any vaft ftretch 

 of ingenuity, ftill lefs an adual appeal to experiment, to perceive that, each time a fmall 

 fcreen was interpofed, the bulb would fuffer a certain depreflion of temperature. 



The 20th, and laft experiment is intended .to confirm the preceding ; with what fuccefs 



will be prefently feen. Another thermometer was placed near the former, but conftantly 



expofed to the ftream of heat : In 4 or 5 minutes it acquired its maximum rife, amounting 



to about a degree and a half. The other therraomer which was fcrecned and expofed alter- 



4 nately 



