to Heat and Illumhiatc OhjeSs, ' 3^9 



receiving heat in the middle, while the outfide remained 

 cold; which would occafidn a partial dilatation. I there- 

 fore cut tli.em into pieces about a quarter of an inch ffiuarcj^ 

 and fet three of thera in. a Aider, fo that; I could move therii 

 behind the fmoked glafs without diftwrbing M\i^ After 

 looking about three or four minutes through one of them, I 

 moved the Aider to the feccnd, and tlien to the third. This 

 kept the glafles fufficicntiy cool : but the difturbance of the 

 alterations proved hurtfol to vifioni, whicir' requires repofe; 

 ^nd, if perchance I flopped a little lorigeu than the proper 

 time, the glafs cracked with a very difagreeable explofion that 

 endangered the ej-e. •' '■; 'ii^'-; ' .. - .c 



No. 10. T*o dark green glafles, 'botii' fmoked,'' that ^. 

 thinner coat might be on each; but the fmoke flill con- 

 -traiftecj-b lifters, though Icfs denfe than before. 



No. ii. To get rid of fmoke entirelv, I ufed two dark 

 green glafles, two very dark green, two pale blue, and one 

 pale green glafs, together. Diftindnefs was wanting, nor wss 

 light fufficientlv interqepted. 



. No. 12. A dark green and a pale blue glafs fmoked. The 

 green glafs cracked. 



No. 13. A pale blue and a dark green glaf^ fmoked. The 

 blue glafs cracked. The eve felt no fenfdtion of !ieat. 

 ■ No. 14. Two pale blue glafles, one fmoked. The firft: 

 glafs cracked. 



It was now fufHcientiy evident that no glafs which flops 

 heat, and therefore receives it, could be prefervcd from 

 cracking, when expofed to the focus of pencils. This induced 

 me to try am application of the darkening apparatus to apother 

 part of the telefcope. 



The place where the rays are leaft condrnred, without in- 

 terfering with tlie reflections of the mirrors, is immediately 

 clofe to the fniall one. I therefore fcrewcd an apparatus to 

 the fpeculum arm^ into which any glafs mioht be plaeed. 



No. J5. A dark green glafs cloft to the fniall fpeculum, and 

 fmoked pale green in the focus of pencils as before. 1 faw 

 remarkably well. 



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

 Vol. VI I. U u mitiht 



