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Updated: June 9, 2025
When the small mirror, on the surface of which the second reflection is effected, is plane, and inclined at an angle of 45° to the axis of the telescope; when the image is reflected laterally, through an opening made near the edge of the tube and furnished with an eye-piece; when, in a word, the astronomer looks definitively in a direction perpendicular to the line described by the luminous rays coming from the object and falling on the centre of the great mirror, then the telescope is called Newtonian.
1st- a brass Sextant of 10 Inches radius, graduated to 15 which by the assistance of the nonius was devisible to 15"; and half of this sum by means of the micrometer could readily be distinguished, therefore-7.5" of an angle was perceptible with this instrument; she was also furnished with three eye-pieces, consisting of a hollow tube and two telescopes one of which last reversed the images of observed objects. finding on experiment that the reversing telescope when employed as the eye-piece gave me a more full and perfect image than either of the others, I have most generally imployed it in all the observations made with this instrument; when thus prepared I found from a series of observations that the quantity of her index error was 8' 45"; this sum is therefore considered as the standing error of the instrument unless otherwise expressly mentioned. the altitudes of all objects, observed as well with this instrument as with the Octant were by means of a reflecting surface; and those stated to have been taken with the sextant are the degrees, minutes, &c shewn by the graduated limb of the instrument at the time of observation and are of course the double altitudes of the objects observed.
"Anyhow, that's as may be.... The beam torpedo tube will just bear nicely in a minute." The white teeth beneath the rubber eye-piece of the periscope showed for an instant in a broad grin. "Won't old man Gedge jump!" "Starboard beam tube ready!" Sir William replaced his eye-glass. A sudden bead of perspiration ran down and vanished into his left eyebrow.
But they are related by mathematical laws, and the apparent haphazard character is only the result of our lack of knowledge of how to interpret the results." He resumed his place at the eye-piece to check over his results. "Walter," he said finally, looking up at me with a twinkle in his eye, "I wish that you'd go out and find me a cat." "A cat?" I repeated.
He continued to look at the various pieces of paper under the microscope. "I find under microscopic examination," he went on, addressing Inez, but not looking up from the eye-piece as he shifted the papers, "that the note you have received, Senorita Mendoza, is written on a rather uncommon linen bond paper. Later I shall take a number of microphotographs of it.
In the seventeenth century telescopes were made of great length, going up to 300 feet. Huyghens also invented the compound eye-piece that bears his name, made of two convex lenses to diminish spherical aberration. But the defects of colour remained, although their cause was unknown until Newton carried out his experiments on dispersion and the solar spectrum.
If the spherical aberration is not correct, the outer rim of the disk will be brighter than the centre when the eye-piece is pushed in, and the centre will be the brighter when it is drawn out. If the curves of the glass are not even all around, the image will appear oval in one or the other position. If there are large veins of unequal density, wings or notches will be seen on the image.
The frames were of the type invented by Mr. Stopford of Moorfields and known by his name. The right eye-piece was fitted with plain glass, as is usual in the case of a blind, or useless, eye. It was very much shattered, but its character was obvious. The glass of the left eye was much thicker and fortunately less damaged, so that I was able accurately to test its refraction.
I watched him curiously, but said nothing, for he was studying something carefully, and I did not want to interrupt his train of thought. Finally he beckoned me over. "Can you make anything out of that?" he asked. I looked through the eye-piece, also. On a sort of fine grating all I could see was a number of strange lines.
"To be sure it does. But what do you look through?" "The eye-piece." "Well, what is the eye-piece?" "A little glass or two lenses." "These glasses or lenses form a microscope, Tom; and through them you look at the tiny image formed in the focus of the great lens or the speculum, whichever you use." "But I thought microscopes were only used to magnify things invisible to the eye."
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