A Description of Helioscopes, and Some Other Instruments

and full. Such a material would not be prone to losing its shape original: "Figure", which all our reflective metals original: "specular Metals" are very likely to do. By using it, it would be possible to shorten the tubes for long telescopes into very short lengths, making them easy to use and manage.

I attempted this with several types of metals made with tin original: ♃, copper original: ♀, iron original: ♂, antimony, and arsenic. However, I found most of these compound metals to be very porous original: "spongy". Consequently, during the final polishing, they acquired a very glaring finish that actually blurred the object with a kind of haziness. This happened especially if polishing powder original: "Putty"; likely tin oxide used for polishing glass and metal. was used to give it a shine. For this purpose, polishing powder must not be used in any way that I know of, as it is very likely to round off the edges of pores or scratches, which contributes greatly to the haziness and blurring of the object.

If I used glass backed with mercury original: "foil'd with Quicksilver", which I found gave the best reflection, I still encountered an inconvenience: a significant part of the light ray was lost due to the double reflection at the un-mirrored surfaces original: "unfoil'd superficies" of the glass. The first reflection comes from the surface of the glass before the light enters it. This not only weakened the ray but also mingled with the other reflection coming from the bottom [the mercury backing], creating a kind of haziness and confusion if the two surfaces of the glass were parallel. If they were not parallel, it introduced some unwanted color, unless it was corrected by an opposing refraction in a second reflecting glass.

This setup is shown in the fifth figure, where ab represents the objective lens, cg is the first reflecting plate (with its thinnest side at c), and d is the second reflecting plate (with its thinnest part toward b). This second plate removes the first refraction from cg and eliminates the colors introduced by the first. The light ray was also weakened much more by the second reflection it suffered at the un-mirrored surface of the glass, caused by the reflection of the air—or ether original: "æther"; a hypothesized substance once thought to fill all space—which is much stronger than that of glass as the light re-enters the air. Besides this, I find that the substance of most glass is so poorly mixed that even in the very best, there are many streaks original: "veinyness" and unequal refraction throughout its parts. Because of this, even though...