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Updated: June 11, 2025
A refracting telescope which has been freed from the effects of chromatic aberration is called achromatic. The principle upon which its construction depends is that by combining lenses of different dispersive power the separation of the spectral colors in the image can be corrected while the convergence of the rays of light toward a focus is not destroyed.
It is, in Professor Lockyer's own words, "little more than a collection of sheds," but within these alleged sheds may be found an excellent equipment of telescopes, both refracting and reflecting, and of all other things requisite to the peculiar study which forms the subject of special research here.
If an advocate is conceited, or vain, or self-important, or if he thinks of producing effects as well for himself as for his client, or if his nature is hard and unadaptive, great abilities display these qualities, instead of hiding them, and they make a refracting medium between a case and the minds of a jury. Mr. Choate was more completely free from them than any able man we ever knew.
I was almost dead with despair. I was in this frame of mind one day, experimenting in my laboratory on a small diamond that stone, from its great refracting power, having always occupied my attention more than any other when a young Frenchman who lived on the floor above me, and who was in the habit of occasionally visiting me, entered the room. I think that Jules Simon was a Jew.
Hence, in passing from the spar to the balsam, the extraordinary ray passes from a less refracting to a more refracting medium, where total reflection cannot occur; while the ordinary ray passes from a more refracting to a less refracting medium, where total reflection can occur.
But its significance lies in those very characteristics. Here was no consuming passion, no bond of sympathy, nothing whatever material or sentimental to act as the refracting medium by which the Hindhead laundry could have been made visible in South Devon. Yet similar phenomena are of constant occurrence.
It is, therefore, called the extraordinary ray. In calc-spar, as just stated, the ordinary ray is the most refracted. One consequence of this merits a passing notice. Pour water and bisulphide of carbon into two cups of the same depth; the cup that contains the more strongly refracting liquid will appear shallower than the other.
No doubt if this had been the case, a perfect telescope could have been produced by properly shaping the object glass. But when Newton had demonstrated that light was by no means so simple as had been supposed, it became obvious that a satisfactory refracting telescope was an impossibility when only a single object lens was employed, however carefully that lens might have been wrought.
The application of the theory of alternate fits of transmission and reflection to explain the colours of thin plates is very simple. Transparency, opacity and colour were explained by Newton on the following principles. Bodies that have the greatest refractive powers reflect the greatest quantity of light from their surfaces, and at the confines of equally refracting media there is no reflection.
Even the stars blazed with a new and unequalled splendor, and, in the absence of a refracting atmosphere, they flamed as bright in the close proximity of the Moon as in any other part of the sky. You can easily conceive the interest with which these bold travellers gazed on the Starry Queen, the final object of their daring journey.
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