280 Dr. Herschel's Investigation oj the Powers 



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. This induced me to try 

 an application of the darkening apparatus to another part of the 

 telescope. 



The place where the rays are least condensed, without inter- 

 fering with the reflections of the mirrors, is immediately close 

 to the small one. I therefore screwed an apparatus to the spe- 

 culum arm, into which any glass might be placed. 



No. 15. A dark green glass close to the small speculum, and 

 smoked pale green in the focus of pencils, as before. I saw 

 remarkably well. 



No. 16. The dark green as before; but, that more light might 

 be admitted, a white smoked glass near the eye. Better than 

 No. 15; but the green glass cracked. 



No. 17. A very dark green and white smoked glass, as 

 before. Very distinct, but the green glass cracked in about six 

 or seven minutes. 



No. 18. A dark blue glass, as in No. 15, and white smoked. 

 This was distinct; and no heat came to the eye. The sun 

 appeared ruddy. 



No. 19. A dark blue and a yellow glass, close together, as in 

 No. 15, and a white smoked one, as before. This was not 

 distinct. _ 



No. 20. A purple glass, as in No. 15, with a white smoked 

 one. This gave the sun of a deep orange colour, approaching 

 to scarlet. It was not distinct. 



No. 21. An orange glass, as in No. 15, with a white smoked 

 one. The colour of the sun was too red. „ 



No. 22. A white smoked glass, as in No. 15, without any 



