A Description of Helioscopes, and Some Other Instruments

...observed by those whose eyesight is weak. However, those who can tolerate a slightly brighter image and wish to see the details more clearly may use a version with only three reflections, like the one in Figure 14, which suits my own eyes best.

Next, this helioscope original: "Helioscope"; a telescope designed for observing the sun safely can be made using reflection alone, without any refraction original: "Refraction"; the bending of light as it passes through a lens. This can be done in the manner shown in Figure 7, where a b represents the concave surface of a piece of black glass with a focus at o. For example, let us suppose this focus is at a distance of forty feet. c d represents a clear plate of glass with two flat surfaces that are not parallel but slightly angled original: "inclining". This ensures that the reflection from the side furthest from the concave surface is cast in a different direction and does not fall at all upon the third reflecting plate, zeta ($ζ$).

Because the wedge-like shape of this transparent glass plate, c d, will cause refraction and therefore a coloration This refers to chromatic aberration, where light incorrectly splits into colors like a rainbow of the ray s, another identical wedge-like plate must be used. This second plate, at a distance such as m p (where the reflection will not hit the plate zeta ($ζ$)), must be fixed so that its thinnest part lies exactly over the thickest part of c d, and its thickest part over the thinnest part of the other. By this means, both the false reflections and the unwanted refractions will be eliminated. From zeta ($ζ$), the rays are reflected to gamma theta ($γ θ$), and from gamma theta ($γ θ$) to the focus at phi ($φ$), and then through the lens z to the eye at kappa ($κ$). I consider this the best design using reflection; however, it could be designed in twenty other ways, which I will not spend more time describing now. Since I have explained these principles, it is easy to imagine a hundred other variations.

This helioscope can be fitted with instruments for measuring the sunspots original Latin: "Maculæ", bright spots original Latin: "faculæ", and hazy patches original Latin: "Nebulæ" visible on the body of the sun. It can also measure the distance they travel over two, three, or ten days, along with changes in their shapes and sizes. However, the diameter of the sun's body is better measured by the instrument described next. Since it will often be necessary to draw these features precisely, the machine original: "Engine" I described on pages 67, 68, and 69 of my book, Animadversions Full title: Animadversions on the First Part of the Machina Coelestis of... Johannes Hevelius (1674), can be used to keep the helioscope constantly pointed at the sun. This will be a great help to an observer who needs to sketch the shapes on paper.