Here the essential peculiarities of the spectrum of each of the stars investigated by HERSCHEL is pointed out, and if we were to use his observations alone to classify these stars into types, we should put _Sirius_ and _Procyon_ into one type of stars which have "all the colors" in their spectra; _Arcturus_ and _Aldebaran_ would represent another group of stars, with a deficiency of yellow and an excess of orange and red in the spectrum; and _[alpha] Orionis_ would stand as a type of those stars with an excess of red and a deficiency of orange. _[alpha] Lyræ_ would represent a sub-group of the first class.
HERSCHEL'S immediate object was not classification, and his observations are only recorded in a passing way. But the fact remains that he clearly distinguished the essential differences of the spectra of these stars, and that he made these observations in support of his statement that the fixed stars, "like the planets, also shine with differently colored light. That of _Arcturus_ and _Aldebaran_, for instance, is as different from the light of _Sirius_ and _Capella_ as that of _Mars_ and _Saturn_ is from the light of _Venus_ and _Jupiter_."
Of course, no special discovery can be claimed for him on these few instances. We can see, however, a good example of the manner in which he examined a subject from every side, and used the most remote evidence exactly in its proper place and time.
_Researches on the Variable Emission of Light and Heat from the Sun._
It is certainly a remarkable fact that HERSCHEL was the first observer to recognize the real importance of the aperture or diameter of a telescope. Before his time it was generally assumed that this element only conditioned the amount of light transmitted to the eye, or, in other words, merely determined the brightness of the image. Hence the conclusion that if an object is sufficiently bright, the telescope may be made as small as desired without loss of power. Thus, in observing the sun, astronomers before HERSCHEL had been accustomed to reduce the aperture of their telescopes, in order to moderate the heat and light transmitted. SCHEINER, it is true, nearly two centuries before the time we are considering, had invented a method for observing the sun without danger, still employing the full aperture. This was by projecting the image of the sun on a white screen beyond the eye-piece, the telescope being slightly lengthened. For special purposes this ingenious method has even been found useful in modern times, though for sharpness of definition it bears much the same relation to the more usual manner of observing, that a photographic picture does to direct vision.