of the prismatic Colours to heat and illuminate Objects. a8 1 



other at the eye. This gave the sun of a beautiful orange colour; 

 but distinctness was totally destroyed. 



No. 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 two 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 two 

 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. 



No. 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 No. 15; after some considerable time, 

 however, this glass also cracked. 



No. 25. I placed a very dark green glass behind the second 

 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 upon the former, 

 with the smoked side towards the eye, that the smoke might 



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