8S4 Method of viewing the Sun with high Powers, 



The tclefcopic experiments, with various darkening glafles applied 4n the eye piece, are 

 numbered and defcribed. Difficulties prefented themfelves, from the heat which pafTed through 

 fome combinations, and the want of light and diftinctnefs in others. The heat which was 

 ".intercepted when the glafles were placed near the focus of the pencils, was alfo found to 

 break them by its partial aftion. To remedy this, one of the glafles was fixed near the 

 fm'all fpeculum, in order that the light might be fpread over a larger furface; but here alfo 

 the heat was too ftrong, and produced the fame inconvenience. The Dodlor therefore at 

 lafl: placed his apparatus clofe behind the eye glafles, as follows in his befl; combination. 



" No. 25. I placed a very dark green glafs behind the fecond eye glafs, that it might 

 be fhelterfcd by both glafles, which in my double eye-piece are clofe together, and of an 

 equal focal length. Here, as the rays are not much concentrated, the coloured glafs re- 

 ceives them on a large furface, and fl;ops light and heat in the proportion of the fquare 

 oi 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 reafon, 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 protected againfl: heat by a paflage of the rays through two furfaces of coloured 

 glafs." 



•' This pofition had moreover the advantage of leaving the telefcope, with its mirrors 

 and glalTes, completely to perform its operation, before the application of the darkeni^ig 

 apparatus ; and thus to prevent the injury which muft be occafioned, by the interpofition 

 «f the heterogeneous colouring matter of the glafles and of the fmoke." 



" No. 26. I placed a deep blue glafs, with a blueifli green fmoked one upon it, as in 

 No. 25, and found the fun of a whiter colour than with the former compofition. There 

 was no difagreeable fenfation of heat } a little warmth might be felt." 



Thefe two are the combinations through which the Do£tor has feen uncommonly well, 

 and in a long feries of very interefling obfervations upon the fun, which will foon be com- 

 municated, the glafles have met with no accident. However, when the fun has confide- 

 rable altitude, he finds it advifeable to leflen the aperture a little in telefcopes, which have 

 fo much light as his ten feet refle£i:or, or (which will give more difl:in<Stnefs) to view the 

 fun earlier in the morning, and later in the afternoon ; becaufe the light intercepted by the 

 atmofphere in lower altitudes, will reduce its brilliancy much more uniformly than it can 

 be foftened, by laying on more fmoke on the darkening glafles. And as few inftruments 

 in common ufe are fo large as that to which this method of darkening has been adapted, 

 he expreflTes his hope that it may be of general ufe in folar obfervations. 



In the fecond paper on the refrangibility of the invtfible rays of the sun, the author fir ft 

 defcribes his apparatus, which is delineated in Plate XIV. where A. B. reprefents a fmall 

 ftand covered with white paper, upon which are drawn five lines parallel to each other, at 

 half an inch diftance afunder, but fo that the firft is only a quarter of an inch from the 

 edge. Thefe lines are interfeded by three others at right angles, the fecond and third of 

 which arc refpedively at 2^ and 4 inches from the firft j i, 2, 3, reprefeat the thermo- 

 » meters 



