HELIOMETER, the name of an instru ment for measuring with particular exact ness the diameters of the heavenly bodies, and especially those of the sun and moon. This instrument is a kind of telescope, consisting of two object-glasses of equal focal distance, placed one of them by the side of the other, so that the same eye-glass serves for both. The tube of this instrument is of a conical form, larger at the upper end, which receives the two object-glasses, than,at the lower, which is furnished with an eye-glass and micrometer. By the construction of this instrument, two distinct images of an object are formed in the focus of the eye-glass, whose distance, depending on that of the two object-glasses from. one another, may be measured with accura cy; nor is it necessary, that the whole disc of the sun or moon should come within the field of view, since, if the images of only a small part of the'disc be formed by each object-glass, the whole diameter may be easily computed by their position with respect to one an other : for if the object be large, the im ages will approach, or perhaps lie even over one another, and the object-glasses being, moveable, the two images may always be brought exactly to touch one another, and the diameter may be com puted from the known distance of the centres of the two glasses. Besides, as this instrument has , a common microme ter in the focus of the eyeglass, when the two images of the sun or moon are made in part to cover one another, that part which is common to both the im ages may be measured with great ex actness, as being viewed upon a ground that is only one half less luminous than itself; whereas, in general, the heaven ly bodies, are viewed upon a dark ground, and on that account are imagined to be larger than they really are. By a small
addition to this instrument, provided it be of a moderate length, Mr. Bonguer, the inventor, thought it very possible to measure angles of three or four degrees, which is of particular consequence in taking the distance of stars from the moon: With this instrument lie found that the sun's vertical diameter, though somewhat diminished by the astronomi cal refraction, is longer than the hori zontal diameter ; and, in ascertaining this phenomenon, he also found, that the upper and lower edges of the sun's disc are not so equally defined as the other parts ; on this account his image appears somewhat extended in the vertical direc tion. This is owing to the decomposi tion of light, which is known to consist of rays differently refrangible in their passage through our atmosphere. Thus the blue and violet rays, which proceed from the upper part of the disc at the same time with those of other colours, are somewhat more refracted than the others, and therefore seem to us to have proceeded from a higher point ; whereas, on the contrary, the red rays proceeding from the lower edge of the disc, being less refracted than the others, seem to proceed from a lower point ; so that the vertical diameter is extended, or appears longer, than the horizontal di ameter.