684 PHILOSOPHICAL TRANSACTIONS. [ANNO 1800. 



However, as I was fully aware of the necessity of providing an apparatus for this 

 purpose, 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 2 of 

 them together. These intercepted full as much light as was necessary ; but I soon 

 found that the eye could not bear the irritation, from a sensation of heat, which 

 it appeared these glasses did not stop. I now took 2 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 considerably more light 

 than the red glasses, they remedied the former inconvenience of an irritation arising 

 from heat. Repeating 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 undertook 

 the investigations which have been delivered under the first 2 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 that pale green light excels in illumination, the method which ought to be 

 pursued in the construction of a darkening apparatus was sufficiently pointed out ; 

 and nothing remained but to find such materials as would give us the colour of the 

 sun, viewed in a telescope, of a pale green light, sufficiently tempered for the eye 

 to bear its lustre. 



To determine what glasses would most effectually stop the red rays, I procured 

 some of all colours, and tried them in the following manner. I placed a prism in 

 the upper part of a window, and received its coloured spectrum on a sheet of white 

 paper. I then intercepted the colours just before they came to the paper, succes- 

 sively, by the glasses, and found the result as follows. A deep red glass inter- 

 cepted all the rays. A paler red did the same. From this, we ought not to con- 

 clude that red glasses will stop the red rays ; but rather, that none of the sun's 

 light, after its dispersion by the prism, remains intense enough to pass through red 

 glasses, in sufficient quantity to be perceptible, when it comes to the paper. By 

 looking through them directly at the sun, or even at day objects, it is sufficiently 

 evident that they transmit chiefly red rays. 



An orange glass transmitted nearly all the red, the orange, and the yellow. It 

 intercepted some of the green ; much of the blue ; and very little of the indigo 

 and violet. A yellow glass intercepted hardly any light, of any one of the colours. 

 A dark green glass intercepted nearly all the red, and partly also the orange and 

 yellow. It transmitted the green ; intercepted much of the blue; but none of the 

 indigo and violet. A darker green glass intercepted nearly all the red ; much of 



