282 Dr. Herschel's Investigation of the Powers 



likewise be protected against heat, by a passage of the rays 

 through two surfaces of coloured glass. 



This position had moreover the advantage of leaving the 

 telescope, with its mirrors and glasses, completely to perform 

 its operation, before the application of the darkening apparatus ; 

 and thus to prevent the injury which must be occasioned, by 

 the interposition of the heterogeneous colouring matter of the 

 glasses and of the smoke. 



No. 26. I placed a deep blue glass with a bluish green 

 smoked one upon it, as in No. 25, and found the sun of a whiter 

 colour than with the former composition. There was no disa- 

 greeable sensation of heat ; though a little warmth might be 

 felt. 



No. 27. I used two black glasses, placed as in No. 25. Here 

 there was no occasion for smoke ; but the sun appeared of a 

 bright scarlet colour, and an intolerable sensation of heat took 

 place immediately. I rather suspect that these are very deep red 

 glasses, though their outward appearance is black. 



In order to have a more sure criterion of heat, I applied Dr. 

 Wilson's thermometer, No. 2, to the end of the eye-piece, 

 where the eye is generally placed. With No. 25, it rose from 

 34 to 37 degrees. With No. 26, it rose from 35 to 46 ; and, 

 with No. 27, it rose, very quickly, from 36 to 0,5 degrees. I am 

 pretty sure it would have mounted up still higher; but, the scale 

 extending only to 100, I was not willing to run the risk of 

 breaking the thermometer by a longer exposure. 



It remains now only to be added, that with No. 25 and 26 I 

 have seen uncommonly well ; and that, in a long series of very 

 interesting observations upon the sun, which will soon be 



