of the prismatic Colours to heat and illuminate Objects. 283 



communicated, the glasses have met with no accident. How- 

 ever, 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 atmo- 

 sphere in lower altitudes, will reduce its brilliancy much more 

 uniformly than we can soften it, by laying on more smoke upon 

 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. 



Slough, near Windsor, 

 March 8, 1800. 



