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year 1775, contrived a clever little instrument which he called a Dynameter: for though when single lenses

Philosophy, and Mathematics; to produce which, he devoted all his time, and almost all the profits of his very extensive trade: in carrying on which, his anxiety was not (like the Razor-maker, who merely made his goods to sell,) to study and contrive how cheap he could make an instrument, and how dear he could sell it; his sole care was to make it as perfect as possible; he spared neither pains nor expense in forming an instrument, or bringing it to perfection; and his insatiable thirst for perfection, almost invariably, produced ultimate success. Without the least ostentation, pride, or reserve in his manners, he was polite, easy, and familiar to all that had business with him.

I have been favoured with the following Anecdote from such a source, that I can vouch for the authenticity of it.

"It was his custom to retire in the evening to what he considered the most comfortable corner in the house, and take his seat close to the kitchen fire-side, in order to draw some plan for the forming a new instrument, or scheme for the improvement of one already made. There, with his drawing implements on the table before him, a cat sitting on one side, and a certain portion of bread, butter, and a small mug of porter placed on the other, while four or five apprentices commonly made up the circle, he amused himself with whistling the favourite air, or sometimes singing the old ballad of,

'If she is not so true to me,

'What care I to whom she be ?

'What care I, what care I, to whom she be !'

and appeared, in this domestic group, contentedly happy. When he occasionally sent for a workman, to give him necessary directions concerning what he wished to have done, he first shewed the

are used, the power of a glass is readily discovered by dividing the focal length of the object-glass by that of the eye-glass, in eye-pieces of the common construction, especially those of a negative focus, it is very difficult to measure in this manner; nor can it be done with with those eyeany accuracy pieces which are made for erect vision with four eyeglasses.

The Dynameter is principally composed of a fine micrometer screw, and a divided plano-convex glass; by means of which the image of the pencil of rays can be completely separated, and the diameter of it known to the greatest nicety. The wheel or head of

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recent finished plan, then explained the different parts of it, and generally concluded by saying, with the greatest good humour, 'Now, see man, let us try to find fault with it;' and thus, by putting two heads together, to scrutinize his own performance, some alteration was probably made for the better. And, whatever expense an instrument had cost in forming, if it did not fully answer the intended design, he would immediately say, after a little examination of the work, Bobs, man! this won't do; we must have at it again :' and then the whole of that was put aside, and a new instrument begun. By means of such perseverance, he succeeded in bringing various mathematical, philosophical, and astronomical instruments to perfection. The large theodolite for terrestrial measurements, and the equal altitude instrument for astronomy, will always be monuments of his fertile, penetrating, arduous, superior genius! There cannot be a lover (especially of this more difficult part) of philosophy, in any quarter of the globe, but must admire the abilities of JESSE RAMSDEN!"

The

the micrometer is divided into two hundred equal parts, and a figure engraven over every fifth division, which is cut rather longer than the others; 1, 2, 3, and so on to 20: but adding an 0 to each figure, it will then read off, 10, 20, 30, and so on to 200. Nonius is divided into 15 towards 0, and engraved 5, 10, 15; and 5 on the contrary side. Each division on the Nonius is equal to one revolution of the micrometer head, and 10 revolutions will bring the edge of the circle round it, and the tenth division on the Nonius to coincide.

The revolutions of the micrometer head will bring the edge of the circle round it, and the division on the Nonius, to coincide at 10: each division, therefore, is equal to the ten thousandth part of an inch.

Applying this little instrument to the eye-glass of a telescope, when adjusted to distinct vision at any distant object, and turning the micrometer head, the emergent pencil will begin to separate; and when the extreme edges are brought into contact, the number of divisions will shew the diameter of it in thousandths of an inch; then reduce the diameter of the Object-glass into thousands, and divide that sum by the diameter of the pencil, the quotient will be the real magnifying power. But as it is requisite for the emergent pencil of rays to be in the focus of the divided glass, a thin transparent piece of ivory, precisely one-tenth of an inch in diameter, is set in the sliding cover, to adjust for that distance, which

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must always be done before it can be used with

accuracy.

When this transparent piece of ivory is brought over the hole in the cover of the dynameter, and appears perfectly round, the Nonius will then be at 0, and is properly adjusted. Five revolutions of the micrometer screw will make a complete separation of the diameter of its aperture, which is one-tenth of an inch and when the opposite sides are brought into contact, the Nonius will coincide at the fifth division of it, which is five two-hundredths of an inch; thus dividing each tenth of an inch into a thousand equal parts. Another method of discovering the magnifying power, is to set the telescope in such a position opposite the sun, that the rays of light may fall perpendicularly on the Object-glass; and the pencil of rays may be received on a piece of paper, and its diameter measured: then, as the diameter of the pencil of rays is to that of the Objectglass, so is the magnifying power of the telescope. Or, thirdly, a thin piece of mother-of-pearl, with a very acute angle two inches long marked thereon, and only one-tenth of an inch at its base marked thereon; the length being divided into ten equal divisions, making a visible line to each division, with a figure over it, these divisions will express or shew the hundredths of an inch: apply this scale to the eye-tube of the telescope, observe where the emergent pencil of rays fills up a certain space at or near

any of the divisions; multiply the diameter of the Object-glass into hundredths on the scale, and the quotient will be the magnifying power.

Neither of these methods will enable you to measure Concaves, but you may measure Concaves by comparing them with Convexes, in the manner I have mentioned to ascertain the relative powers of Lenses.

Before any of these methods of finding the magnifying power be made use of, remember to look through the tube, and observe carefully if some of the Object-glass be not cut off, and part of the original pencil intercepted by the Stops in the Tube, or Eye-pieces, &c., which frequently curtail the light of the Object-glass as effectually as a cap over the end of it would.

This is a very common trick, and will render your calculation on the whole aperture erroneous.

To ascertain whether any of the Object-glass is cut off by the Stop in the Eye-tube-adjust the Telescope to distinct Vision-then take out the Eye-glass, put your finger on the edge of the outside of the Objectglass, and look down the tube-if you can see the tip of your finger just peeping over the edge of the Object-glass none is cut off.

In all cases, the Magnifying power of Telescopes, or Microscopes, is measured by the proportion of the diameter of the original pencil, to that of the pencil which enters the Eye.

The following Observations on Deep Lenses were

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