688 PHILOSOPHICAL TRANSACTIONS. [ANNO 1800. 



vent the injury which must be occasioned, by the interposition of the heterogeneous 

 colouring matter of the glasses and of the smoke. 



N° 26. I placed a deep blue glass with a bluish green smoked one on it, as in 

 N° 25, and found the sun of a whiter colour than with the former composition. 

 There was no disagreeable sensation of heat; though a little warmth might be felt. 

 N° 27. I Uvsed 2 black glasses, placed as in N° 25. Here there was no occasion 

 for smoke; but the sun appeared of a bright scarlet colour, and an intolerable Sen- 

 sation 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 for heat, I applied Dr. Wilson's thermo- 

 meter, N° 2, to the end of the eye-piece, where the eye is generally placed. With 

 N° 25, it rose from 34 to 37 degrees. With N° 26, it rose from 35 to 46; and, 

 with N° 27, it rose, very quickly, from 36 to 95 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 N° 25 and 26 I have seen uncommonly 

 well ; and that, in a long series of very interesting observations on the sun, which 

 will soon be communicated, the glasses have met with no accident. However, when 

 the sun is at a considerable altitude, it will be advisable to lessen the aperture a 

 little, in telescopes that have so much light as my 10-feet reflector; or, which will 

 give us more distinctness, to view the sun earlier in the morning, and later in the 

 afternoon; for, the light intercepted by the atmosphere in lower altitudes, will 

 reduce its brilliancy much more uniformly than we can soften it, by laying more 

 smoke on our darkening glasses. Now as few instruments in common use are so 

 large as that to which this method of darkening has been adapted, we may hope 

 that it will be of general utility in solar observations. 



XIV. Experiments on the Ref Tangibility of the Invisible Rays of the Sun. By 

 JVm. Herschel, LL. D. t F. R. S. p. 284. 



In that section of my former paper which treats of radiant heat, it was hinted, 

 though from imperfect experiments, that the range of its refrangibility is probably 

 more extensive than that of the prismatic colours; but having lately had some 

 favourable sun-shine, and obtained a sufficient confirmation of the same, it will be 

 proper to add the following experiments to those which have been given. I pro- 

 vided a small stand, with 4 short legs, and covered it with white paper. (See fig. 

 14, pi. 11). On this I drew 5 lines, parallel to one end of the stand, at half an 

 inch distance from each other, but so that the first of the lines might only be 4- of 

 an inch from the edge. These lines I intersected at right angles with 3 others; 

 the 2d and 3d of which were respectively at 2^- and at 4 inches from the first. 

 The same thermometers that have before been marked N° J, 2, and 3, mounted 

 on their small inclined planes, were then placed so as to have the centres of the 



