Page:In the high heavens.djvu/309

 be different in the middle observation of the three, from that which it was both in the first and in the third. Every practical astronomer knows this is a research which admits of being made with great precision. The angular distance of the two star-like points is an element which the micrometer will indicate accurately to the fraction of a second, and if there were a displacement amounting to one hundredth part of that which corresponds to the horizontal refraction of our own atmosphere, the refractive capacity of the comet would be quite unmistakable.

By observations of this class it has been shown that there is little or no appreciable refractive power in one of these