to Heat and Illuminate OljeHs. 335 



glaflfes are generaliv placed. Being thus left without refource, 

 I made ufe of red glalfes; but was by no means fatisficd with 

 their performance. My not being better prepared, as it hap- 

 pened, was of no confequence; the weather proving totally 

 unfavourable for viewing the fun at the time of the tranfit. 

 However, as I was fully aware of the neceffily of providing 

 an apparatus for this purpofcj fince no method that was in 

 ufe could be applied to my tclefcopes, I took the firft oppori- 

 Uinitv of beginniiig my trials. 



The inftrument I wifhed to adapt for folar infpe6lion, was a 

 Newtonian refleftor, with 9 inches aperture; and my aim 

 was to ufe the whole of it open. 



I began with a red glafs; and, not finding it to flop light 

 enough, took two of them .together, Thefe intercepted full 

 as much light as was ncce'fary; but I foon found that the 

 eye could not bear the irritation from a fenfation of heat, 

 which it appeared thefe glalTes did not (lop. 



I now took two green glallcs : but found that they did not 

 intercept light enough. I therefore finoked one of them; and 

 it appeared that, notwithlianding they now ftill tranfmitted 

 confiderably more light than the red glaflfes, they remedied 

 the former inconvenience of an irritation arifmg from heat. 

 Repealing thefe trials feveral times, I conftantly found thp 

 fame refult ; and, the fun in the firil cafe being of a deep red 

 colour, I furmifed that the red-making rays, tranfmitted 

 through red glafl'es, we're more efficacious in raifing a fenfa- 

 tion of heat than thofe which paOed through green, and 

 which caufed the fun to look grecnilh. In cnnle(]uence of 

 this furmife, I undertook the invefligations which liave bee^ 

 flelivered under the two firft heads. 



As foon as I was convinced that the red light of the fuii 

 pught to be intercepted on account of the heat it occafions, 

 and that it might alfo be fafcly fct afide, fince it was now 

 proved that pale green light excels in illumination, the metliod 

 ■yvhich ought to be purfued in the confiruftion ot a dark- 

 ening apparatus was fufficiently pointed out; and liOthing 

 remained but to find fach materials as would give us the 

 colour of the fun, viewed in a telefcopc, of a pale green light, 

 ftifficienlly tempered for the eye to btiir its k.ftre. 



