A Description of Helioscopes, and Some Other Instruments

[soot from a] lamp, though not to such a great degree. The image [original: Figure] on paper, or a smooth white surface, is not magnified enough, nor are the differences between shadows very distinct; however, that method works well enough if the surface is very smooth and the object is magnified by a magnifying glass original: "Hand Glass". The method using a narrowed aperture The opening through which light enters the telescope. is the worst of all. This is because of a certain property of light not yet noticed by writers on optics: the edges of objects seem ragged. I hinted at this in my Animadversions, page 35 Refers to Hooke's 1674 work, Animadversions on the First Part of the Machina Coelestis of... Hevelius, where he discussed the limitations of certain optical instruments., and I shall soon say much more about it, as the entire foundation of optics depends on it.

The method of expanding the image of the sun using the eyepiece original: "Eye Glass" seems to me the best of all the others. However, it is prone to distort [original: vitiate] the image and introduce some unwanted color Hooke is referring to chromatic aberration, where lenses fail to focus all colors to the same point.. It does not provide the finest distinctions of light and shadow without some color, haziness, and dimness.

The lenses of this HELIOSCOPE An early instrument designed by Hooke for observing the sun safely. may be made using either refracting or reflecting spherical glass. The best way to capture a wide field of view [original: large Angle] is to use refracting lenses for both the objective The primary lens that collects light from the object. and the eyepiece. However, the best way to view a small area—and to avoid haziness, dimness, and unwanted colors—is through reflection, either in part or in whole. That is, one can make only the objective lens using reflection and the eyepiece using refraction, or use reflection for both the objective and the eyepiece, having no refraction at all. I have shown the various ways of doing this in the accompanying table [illustration], where I have illustrated ten different ways of arranging the lenses to be fit for the intended use.

The first method, shown in the first figure, uses a sixty-foot objective lens condensed into a twelve-foot tube with the help of four different reflecting plates placed between the objective and the eyepiece. I conducted and showed the experiment for this before the Royal Society The premiere scientific institution of the era. during several of their public meetings at Arundel House The temporary meeting place of the Royal Society after the Great Fire of London. in 1668, and a record of it remains in their register.

As I showed then, this would be of exceedingly great use for all types of perspectives Small handheld telescopes or spyglasses. and telescopes, if only we could find a good material that would make the reflections very strong...