686 PHILOSOPHICAL TRANSACTIONS. [ANNO 1800. 



scope; and found that it made the sun appear of a scarlet colour. They transmitted 

 some heat; and distinctness was greatly injured. N° 4. I used a very dark green 

 glass, to stop heat ; and behind it, or towards the eye, I placed a red glass, to stop 

 light. The first glimpse I had of the sun, was accompanied with a sensation of 

 heat; distinctness also was materially injured. N° 5. I used a dark green and a 

 pale red; but, the sun not being sufficiently darkened, I smoked the red glass, and, 

 putting a small partition between the two, placed the smoke towards the green 

 glass. This took off the exuberance of light; but did not remedy the inconve- 

 nience arising from heat. 



N° 6. I used 2 pale green glasses; smoking that next to the eye, and placing it 

 as in N° 5, so that the smoke might be inclosed between the two. This acted in- 

 comparably well; but, in a very short time, the heat which passed the first glass, 

 (though not the second, for I felt no sensation "of it in the eye) disordered the 

 smoke, by drawing it up into little blisters or stars, which let through light; and 

 this composition therefore soon became useless. N°7- I used 2 dark green glasses, 

 one of them smoked, as in N 5. These also acted well; but became useless, for 

 the reason assigned in N° 6, though somewhat less smoke had been required than 

 in the former composition, I felt no heat. N° 8. I used one pale green, with a 

 dark green smoked glass on it, as in N° 5. It bore an aperture of 4 inches very 

 well, and the smoke was not disordered; but when all the tube was open, the pale 

 green glass cracked in a few minutes. N° 9. Placing now a dark green before a 

 smoked green, I saw the sun remarkably well. In this experiment, I had made a 

 difference in the arrangement of the apparatus. The cracking of the glasses, I 

 supposed might be owing to their receiving heat in the middle, while the outside 

 remained cold; which would occasion a partial dilatation. I therefore cut them into 

 pieces about a quarter of an inch square, and set 3 of them in a slider, so that I 

 could move them behind the smoked glass, without disturbing it. After looking 

 about 3 or 4 minutes through one of them, I moved the slider to the 2d, and then 

 to the 3d. This kept the glasses sufficiently cool ; but the disturbance of the alte- 

 rations proved hurtful to vision, which requires repose; and if perchance I stopped 

 a little longer than the proper time, the glass cracked, with a very disagreeable ex- 

 plosion, that endangered the eye. 



N° 10. Two dark green glasses, both smoked, that a thinner coat might be on 

 each; but the smoke still contracted blisters, though less dense than before. N° 

 1 1 . To get rid of smoke entirely, I used 2 dark green glasses, 2 very dark green, 

 2 pale blue, and 1 pale green glass, together. Distinctness was wanting; nor was 

 light sufficiently intercepted. N° 12. A dark green and a pale blue glass, smoked. 

 The green glass cracked. N° 13. A pale blue and a dark green glass, smoked. 

 The blue glass cracked. The eye felt no sensation of heat. N° 14. Two pale blue 

 glasses, one smoked. The first glass cracked. 



It was now sufficiently evident, that no glass which stops heat, and therefore 

 receives it, could be preserved from cracking, when exposed to the focus of pencils. 



