# The Method of Using the Catoptric Microscope and Telescope

# English Text

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THE
Method of Using the
CATOPTRIC MICROSCOPE
AND
TELESCOPE.
Made and sold by JOSEPH JACKSON,
MATHEMATICAL INSTRUMENT-MAKER, in Angel-
Court in the Strand, London.
The Use and Advantage of the CATOPTRIC MICRO-
SCOPE, from the Philosophical Transactions, No. 442.

"Though Microscopes composed of Refracting Glasses only, have been vastly improved as to their Effects of Magnifying; yet they have been attended with such great Inconveniences that their Application to many Arts, in which they might be very convenient, is not so common as might be expected, and Mankind have reap'd but a small part of the Advan- tage obtainable from so surprizing and useful an Instru- ment. Among the Inconveniences mentioned, these are the most considerable.

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I. That in order to magnify greatly, it's necessary the Object-Glass be a Portion of a very minute Sphere, whose Focus being very short, the Object must be brought exceeding near; it will therefore be shaded by the Microscope, and not visible by any other Light than what passes through itself; in this Case therefore, Opake Objects will not be seen at all. II. Objects illuminated this way, may be rather said to eclipse the Light than to be truly seen, little more being exactly represented to the Eye than the Out-line; the Depressions and Elevations within the Out-line ap- pearing like so many Lights and Shades, according to their different Degree of Thickness or Transparency; though the contrary happens in ordinary Vision, in which the Lights and Shades are produced by the dif- ferent Exposure of the Surface of the Body to the inci- dent Light. III. Small Parts of large Objects cannot easily be ap- plied to the Microscope, without being divided from their Wholes, which in the Case of Vivisection defeats the Experiment, the Part dying, and no more Motion being observed therein. IV. The Focus in the Dioptric Microscope being so very short, is exceeding nice, the least Deviation from it rendering, Vision turbid; therefore a very small Part of an irregular Object can be seen distinctly this way. To remedy these Defects I have contrived a Micro- scope on the Model of the Newtonian Telescope. I

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shall say nothing of the Effects of this Instrument, ex- cepting that it magnifies from the Distance of 9 to 24 Inches, having the Honour of shewing this Instrument to this learned Society." This Instrument, besides its Use as a Microscope, is con- vertible into a Gregorian Telescope, by changing the small Speculum for one of a different Focus. It has this Con- venience, that some Peculiarities in the Form of the great Speculum, necessary to make it succeed as a Microscope, give it an Advantage, both in Light and Distinctness, over most of the Reflecting Telescopes hitherto made. The Manner of using this Instrument as a Microscope is this: When the Instrument is fix'd on its Stand, the small Speculum A must be thrust home in its Slider, in the Inside of the Mouth of the Instrument B, and the Screw C turned till the Index D cuts one of the Numbers at M; then the Mouth of the Instrument B must be re- mov'd from the Object the Distance in Inches express'd by that Number, and the Instrument must be directed to- wards the Object, by looking thro' the Hole in the great Speculum; the Tube that holds the Eye-Glasses being taken off, and adjusting it by means of the two Racks E and E in such manner, that the Image of the Object may be vi- sible in the middle of the little Speculum: Then the Tube F1 or F2 holding the Eye-Glasses, must be screw'd on, and the small Eye-Hole in the little Brass Plate, which turns on a Pivot, applied. N.B. As the same Adjustment of the Speculum will not suit all Eyes, the Screw C must be turned round to the Right or Left a little, till the Object appears distinct.

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The Use of the TELESCOPE The Instrument must be put together as for the Mi- croscope, but the larger of the small Speculums must be placed in the Slider in the Inside of the Mouth of the Instrument B, and the Screw C turned till the Mark G on the Index comes to the Mark T, which is the Po- sition of it for all Objects at a great distance. And the aforesaid little Brass Plate, containing the small Eye-Hole, must be remov'd, that the larger Hole may be uncover'd. The Instrument must be directed towards the Object by means of the Sights HH. When the Telescope is to be fitted for the Observation of the Sun, the dark Glass K is to be screw'd over the Eye-Hole at the End L. N and N are two Screws for charging or discharging the Rack Motions.