II. Description of a New Darkening Glass for Solar Observa- 

 tions, which has also the property of polarising the whole 

 of the transmitted Light. By David Brewster, LL. D* 

 F. R. S. Lond. & Edin. & F. A. S. E-. 



[Read 1st May 1815.] 



T will be readily admitted by every person who has been 

 accustomed to solar observations, that an apparatus for di- 

 minishing the intensity of the sun's light, without distorting 

 or colouring the resulting image, is still a desideratum in Prac- 

 tical Astronomy. 



Dr Herschel is the only person who has given any degree 

 of attention to this subject. When he applied his powerful 

 telescopes to examine the surface of the sun, he found that the 

 ordinary method of attenuating the light by smoked or colour- 

 ed glasses, was of no avail ; and it was in the prosecution of his 

 experiments for determining the relative advantages of differ- 

 ently coloured glasses, or of combinations of differently colour- 

 ed glasses, that he was conducted to those splendid discoveries 

 respecting the invisible rays, which have formed an epoch both 

 in Chemistry and Optics. 



Vol. VIII. P. I. D The 



