VOL. XC.] PHILOSOPHICAL TRANSACTIONS. 6§7 



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 speculum arm, into which any glass might 

 be placed. N° 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. N° l6\ The 

 dark green as before; but, that more light might be admitted, a white smoked 

 glass near the eye. Better than N° 15; but the green glass cracked. N° 17. A 

 very dark green and white smoked glass, as before. Very distinct, but the green 

 glass cracked in about 6 or 7 minutes. N° 18. A dark blue glass, as in N° 15, 

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

 appeared ruddy. N° 1 9. A dark blue and a yellow glass, close together, as in N° 

 15, and a white smoked one, as before. This was not distinct. N° 20. A purple 

 glass, as in N° 15, with a white smoked one. This gave the sun of a deep orange 

 colour, approaching to scarlet. It was not distinct. N° 21. An orange glass, as 

 in N° 15, with a white smoked one. The colour of the sun was too red. N° 22. 

 A white smoked glass, as in N° 15, without any other at the eye. This gave the 

 sun of a beautiful orange colour, but distinctness was totally destroyed. 



N° 23. The heat near the small speculum being still too powerful for the glasses, 

 I had a bluish dark green glass made of a proper diameter to be inclosed between 

 the 2 eye-glasses of a double eye-piece. All glass I knew would stop some heat; 

 and was therefore in hopes that the interposition of this eye-glass would temper 

 the rays, so as in some measure to protect the coloured glass. In the usual place 

 near the eye, I put 2 white glasses, with a thin coat of pitch between them. These 

 glasses, when looked through by the natural eye, give the sun of a red colour; I 

 therefore entertained no great hopes of their application to the telescope. They 

 darkened the sun not sufficiently; and, when the pitch was thickened, distinctness 

 was wanting. N° 24. The same glass between the eye-glasses, and a dark green 

 smoked glass at the eye. Very distinct. This arrangement is preferable to that of 

 N° 15; after some considerable time however this glass also cracked. N° 25. I 

 placed a very dark green glass behind the 2d eye-glass, that it might be sheltered 

 by both glasses, which in my double eye-piece are close together, and of an equal 

 focal length. Here, as the rays are not much concentrated, the coloured glass 

 receives them on a large surface, and stops light and heat, in the proportion of 

 the squares of its diameter now used, to that on which the rays would have fallen, 

 had it been placed in the focus of pencils. And, for the same reason, I now also 

 placed a dark green smoked glass close on the former, with the smoked side to- 

 wards the eye, that the smoke might also be protected against heat, by a passage 

 of the rays through 2 surfaces of coloured glass. This position had also the ad- 

 vantage of leaving the telescope, with its mirrors and glasses, completely to perform 

 its operation, before the application of the darkening apparatus; and thus to pre- 



