It will be readily admitted by every person who has been accustomed to solar observations, that an apparatus for diminishing the intensity of the sun's light, without distorting or colouring the resulting image, is still a desideratum in Practical 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 coloured glasses, was of no avail; and it was in the prosecution of his experiments for determining the relative advantages of differently coloured glasses, or of combinations of differently coloured glasses, that he was conducted to those splendid discoveries respecting the invisible rays, which have formed an epoch both in Chemistry and Optics.