»74 ® r < Herschel's Investigation of the Powers 



My not being better prepared, as it happened, was of no conse* 

 quence ; the weather proving totally unfavourable for viewing 

 the sun at the time of the transit. However, as I was fully 

 aware of the necessity of providing an apparatus for this pur- 

 pose, since no method that was in use could be applied to my 

 telescopes, I took the first opportunity of beginning my trials. 



The instrument I wished to adapt for solar inspection, was a 

 Newtonian reflector, with 9 inches aperture ; and my aim was, 

 to use the whole of it open. 



I began with a red glass ; and, not finding it to stop light 

 enough, took two of them together. These intercepted full as 

 much light as was necessary; but I soon fbund that the eye 

 could not bear the irritation, from a sensation of heat, which it 

 appeared these glasses' did not stop. 



I now took two green glasses ; but found that they did not 

 intercept light enough. I therefore smoked one of them ; and it 

 appeared that, notwithstanding they now still transmitted con- 

 siderably more light than the red glasses, they remedied the 

 former inconvenience of an irritation arising from heat. Re- 

 peating these trials several times/I constantly found the same 

 result ; and, the sun in the first case being of a deep red colour, 

 I surmised that the red-making rays, transmitted through red 

 glasses, were more efficacious in raising a sensation of heat, 

 than those which passed through green, and which caused the 

 sun to look greenish. In consequence of this surmise, I under- 

 took the investigations which have been delivered under the 

 two first heads. 



As soon as I was convinced that the red light of the sun 

 ought to be intercepted, on account of the heat it occasions, and 

 that it might also be safely set aside, since it was now proved 



