A Description of Helioscopes, and Some Other Instruments

...there were no visible veins appearing in the body of the glass; and although both surfaces were very accurately shaped original: "figured" and polished, there was still a kind of dimness cast over the objects by the rays passing through those glasses. However, this was not the case with all of them, for I found some that met my expectations very well. I am inclined to believe that if a pot of glass were made specifically for this purpose, using a method I know, its body could be made perfectly clear, uniform, and transparent, without air bubbles original: "blebs", veins, or sand grains. I design to experiment original: "experience" further with this when I have the leisure and opportunity.

But this is only an aside regarding the shortening of telescope tubes for observing the Moon, planets, and other objects. It is not at all relevant to our present purpose of making a Helioscope. In a Helioscope, we only use the reflection from the first surface original: "superficies"; the outermost face of the glass. of the glass. Our main aim and design is the reduction original: "loss" of the strength and brightness of the light rays, rather than preserving or increasing their intensity and vibrancy.

Therefore, for this use, the best materials I have yet found are black glass, black marble, and glass of antimony original: "Glass of Antimony"; a dark, semi-transparent glass made by roasting and melting antimony ore.. Because these substances are very dark and opaque, they reflect only a very small portion of the rays that fall upon them, and none of the rays that penetrate into them (especially if the material is thick). Being made of very hard and durable substances, they are capable of receiving a very exquisite and exact polish, and they are well-qualified to retain it without being damaged by the air or ordinary wiping.

However, in making these glasses for long telescopes, very great care and diligence must be used to make them perfectly flat. This is even more important the closer they are placed to the objective lens original: "Object-Glass" and the further they are from the eyepiece original: "Eye-Glass". A small error at a great distance from the eye is vastly magnified, whereas a larger error becomes unnoticeable if it is near the eye.

Let a b, in the first figure, represent a sixty-foot glass whose focus is at o. Let a c d e f o, and b g h i k o, represent the two side rays of the beam original: "pencil" of light. This beam, by the four reflecting surfaces (γ n, δ θ, ε ι, ζ κ), is broken into five shorter lengths:

• a n answering to c d
• γ θ to g h
• δ ι to d e
• ε κ to h i
• ζ o to e f
• and κ o to i k