fo Heal and lUummate Objeds. 331 



trsy double eye-piece arc clofe together, and of an equal focal 

 length. Here, as the ravs are not much concentrated, the 

 coloured glafs receives them on a large furface, and (lops light 

 and heat in the proportion of tiie i"L|uares of its diameter now 

 ufed, to that on which the rays would have fallen had it 

 been placed in the focus of pencils; and for the fame rea- 

 fon I now alfo placed a dark green fmoked glafs clofe 

 upon the former, with the fmoked fide towards the eye, that 

 the fmoke might likewife be proteAed againlt hedt by a paf- 

 iageof the rays through two furfaccs of coloured glafs. 



This pofition had moreover * he advantage of leaving the 

 Jclefcope, with its mirrors and glafles, completely to perform 

 Its operation before the application of the darkening appa- 

 ratus ; and tluis to prevent the injury which muR be occa- 

 fioned by the interpofition of the heterogeneous colouring 

 Inatter of the glafles and of the fmoke. 



No. 26. I placed a deep blue glafs with a blueifli green 

 fmoked one upoii it, as iri No. 25, and found the fun of a 

 whiter colour than with the former compofition. There was 

 no difagreeable fenfation of heat, though a little warmth 

 might be telt. 



No. 27. I ufed two black glafles, placed as in No. 25. 

 Here there was no occafion for fmoke ; but the fun a]3- 

 peared of a bright fcarlet colour, and an intolerable fenfation ' 

 of heat took place immediately. I rather fufpcdrt that thefe 

 are very deep red glafles, though their outward appearance is 

 black. 



la order to have a more furie criterion of heat, 1 applied 

 Dr. Wilfon's thermometer, No. 2, to the end of the eye- 

 piece, where the eye is generally placed. \Viih No. 25, it 

 rofe from 34 to 37 degrees; with No. 36, it rofe from 35 

 to 46; and with No. 27, it rofe very quickly from 2,^ to 95 

 degrees. I am pretty fure it would have mounted up flill 

 higher; but, the fcale extending only to 100, 1 was not 

 willino; to run the rilk of breakint;" the thennonieter bv a 

 lunger expofure. 



It remains now only to bt; added, that with No. 25 and 



26 1 have feen uncommonly well; and that, in a long feries 



■ : verv intereftiiig obfervations upon the fun, which will 



IJ u 2 foon 



