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Convex small metal, of the same magnifying power as the Concave, which he may procure for a Guinea. As honest ISAAC NEWTON wrote in page 500 of vol. x. of the Phil. Trans. "This is to be decided
not by Discourse, but by a new trial of the Experiments."
HOW TO ADJUST, OR SET A TELESCOPE, AND OF THE FIELD OF VIEW.
FROM the want of knowing how to adjust a Telescope to distinct Vision for different Distances, I have not been much surprised when I have heard some people complain most clamorously, that they never met with a Glass through which they could see distinctly; and others, that their Eye is always so strained by looking through a Telescope that they are afraid to use one.
However, to look through a good Glass when it is accurately Adjusted, I believe is very little, if any, more fatigue to the Eye, than it is to look with as earnest attention, at the same Object, for the same length of time with the naked Eye.
It should be explained to those who have not been accustomed to use Telescopes, that if every part of the Instrument is perfect, and perfectly cleanthat if objects do not appear perfectly distinct, and sharply defined, that fault must arise either from the various parts of the Telescope not being properly adjusted to each other, or to the Instrument not being adjusted to the Eye of the person observing.
Each person ought to set a Telescope for his own Eye, for almost every Eye, even of people of the same age, has its peculiar focus.
Persons unaccustomed to adjust a Telescope, are often unable to do so with that degree of nicety which is needful to produce perfectly distinct vision : and it is extremely difficult for another person to do so for them, however well acquainted with the usual peculiarities of the Eye at various Ages:-thus the most interesting parts of Telescopic Exhibitions are often seen but very imperfectly.
To give some idea of the Focus, Opticians sometimes draw a line round the Tube, where the Telescope is most distinct for a Common Eye at the distance commonly required.
As the Reader may have observed, that the Spying Glasses which are in use at Watering Places and at Sea-have a mark on their tube which is called the place to set it to-very few persons have any idea that every variation in the distance of the Object, or the Age of the person, requires a variation of the adjustment of the Glass.
When You use a Telescope,-hold the Outer tube in one hand, and the Inner with the other handlook through the centre of the Glass at the Object you wish it to shew you, and ADJUST IT patiently and precisely:-thus,-press the Eye-tube towards the Object-Glass, Vision will gradually increase in distinctness as the Eye-Glass approaches its proper
distance from the Object-Glass, and when there, the Object will be seen perfectly and sharply defined if the Eye-tube be put in beyond the proper distance, the object will again become indistinct, and in that case, the Eye-tube must be withdrawn again-a very little practice, will enable a person easily to obtain the precise point at which the most perfect distinctness can be obtained. This is a much better way of adjusting a Glass than to put it up to the Eye, and then pull out the Inner tubeby which act, if the tube does not slide regularly, or is shorter than you expect, it may suddenly slip out, and strike your Eye, and plant a Cataract.
The greater the Magnifying power of a Glass, the greater nicety is required in adjusting it.
If You wish to see any thing further off, or - for each variation of distance, a corresponding variation of ADJUSTMENT is required; i. e. of the distance of the Eye-piece from the ObjectGlass-which must be diminished, in the proportion that the distance of the Object is increased. This caution is quite necessary-I have met with many persons who have condemned a Glass because they could only see some objects distinctly with it, and for others they found it useless-merely, because they had not been told, that every variation of the Distance of the Object, requires a corresponding variation in the ADJUSTMENT.
More Glasses have been condemned for the want
of this knowledge than from any other cause-and more Eye and Object-Glasses have been spoiled. Those who are not aware of it suppose that when they turn their Glass to an Object to which it is not adjusted that its Glasses want wiping, and they keep rubbing, till in a little time they render them about as unfit to look through as Ground Glass.
Sometimes a Film or Fog forms between the Object-Glasses, or, as the Optical phrase is, the Glasses sweat:"-when this happens, they must be taken out of their cell and wiped with a bit of soft Leather or of very fine Silver Paper-but never do this but when it is absolutely needful, and then, take care to replace them in the same position; it is seldom requisite oftener than once or twice in a Year. Nor wipe the Object or Eye-Glass except they really require it as often as you wipe them-you scratch them a little.
To See an Object distinctly at any given distance, The longer and older the Sight of the Person, the longer the tube must be drawn out-Thus-if a person of 20 years of Age, who has a common Eye, has adjusted a Glass, for distinct vision at the distance of 60 yards—and wishes to set it so that a person of 40 or 50 years of Age, who uses Convex Spectacles of 36 or 30 Inches focus, may see as distinctly with it an object at the distance of 60 yards he must pull out the tube about the eighth
of an Inch further, more or less, as the Eye is