Method of viewing the Sun through large Tele/copes. ^923 



greater or lefs momenta are likely to become equally unfit for the impreflions of fight. 

 Whereas in radiant heat there may be no fuch limitation to the momentum of its particles. 

 From the powerful cfFedls of a burning lens, however, the information is gathered, that the 

 momentum of terreftrial radiant heat is not likely to exceed that of the fun ; and that con- 

 fequently the rcfrangibility of calorific rays cannot much extend beyond that of calorific 

 light. Hence alfo it is inferred, that the invifible heat of red hot iron gradually cooled 

 till it ceafes to fliine, has the momentum of the invifible rays, which in the folar fpcftruni, 

 viewed by day light, go to the confines of red; and this will afford an eafy folution of the 

 refleftion of invifible heat by concave mirrors. 



The refults of the foregoing inveftigation, originally fuggefted by the phenomena of 

 vifion by telefcopes direfted to the fun, became ufeful in their turn to dire£l the procelTes, 

 by which a method might be obtained of viewing that luminary -with telescopes of large aper- 

 tures and high magnifying powers. The focal heat in the large telefcopes ufed by the Dodtor, 

 •was fufficient fpeedily to break the darkening flips of wedge-formed glafles, commonly ufed' 

 with achromatic telefcopes. It was a Newtonian reflector of nine inches aperture, 

 which he wiftied to adapt for folar infpe£tion, and his aim was to employ the whole furfacc 

 of the fpeculum. Two red glafles intercepted full as much light as was neceflary ; but the 

 eye could not bear the irritation arifing from a fenfe of heat. Green glafles, one of which 

 was fmoked, ftill gave a brighter illumination than the red, but they remedied the incon- 

 venience of the heat. Various trials for intercepting the red, and other rays from a prifm, 

 with difi'erently coloured glafles, were made ; of which the particulars are given, and alfo 

 of the effefts of a coating of fmoke and of pitch, and of coloured fluids, the latter of which 

 when denfe enough to ftop much light, were not found fufliciently pure to be ufed. 



Thefe lafl: trials, however, were not fufficient to afcertain the very eflential particular of 

 difl:in£tnefs afforded by thefe feveral media. It was neceflary to try the feveral glafl"e$ and 

 combinations in the aftual inftrument, previous to the account of which the Do£lor dc- 

 fcribes an eafy way of uniformly fmoking glafl"es, which I fhall proceed to copy, 



" With a pair of warm pliers take hold of the glafs, and place it over a candle at a 

 fufficient diftance not to contract fmoke. When it is heated, but no more than ftill to 

 permit a finger to touch the edges of it, bring down the glafs at the fide of the flame as low 

 as the wick will permit, which muft not be touched. Then, with a quick vibratory mo- 

 tion, agitate it in the flame from fide to fide ; at the fame time advancing, and retiring it 

 gently all the while. By this method you may proceed to lay on fmoke to any required 

 darknefs. It ought to be viewed from time to time, not only to fee whether it be fuffi- 

 ciently dark, but whether any inequality may be perceived ; for if that fliould happen, it 

 will not be proper to go on. 



" The fmoke of fealing wax is had : that of pitch is worfe. A wax candle gives a good 

 fmoke : that of a tallow candle is better. As good as any I have hitherto met with, is the 

 fmoke of fpermaceti oil. In ufing a lamp you may alfo have the advantage of an even 

 flame extended to any length." 



T t 2 The 



