Of the Reflecting Telescope.

THE great difficulty of managing refracting telescopes when of great length, and the impossibility of obtaining a higher magnifying power than 500 times, even by making them 600 feet long, stimulated philosophers to attempt a different mode of constructing telescopes, which they have effected by applying the principle of reflection, instead of that of direct vision. By this means instruments of a much shorter length answer the purpose much better; for a reflecting telescope, 6 feet long, will magnify as much as a refracting one 100 feet long.

Gregorian Telescope.

THE first telescope of the reflecting kind, which was found to answer the purpose, was invented by Dr. James Gregory, a Scotsman, about the year 1661.

The instrument which he invented is represented by the following figure.

At the bottom of the great tube TTTT is placed the large concave mirror DUVF, whose principal focus is at m; and in its middle is a round hole P, opposite to which is placed the small mirror L, concave toward the great one; and so fixed to a strong wire M, that it may be moved farther from the great mirror, or nearer to it, by means of a long screw on the outside of the tube, keeping its axis still in the same line P mn, with that of the great one. Now, since in viewing a very remote object, we can scarcely see a point of it but what is at least as broad as the great mirror, we may consider the rays of each pencil, which flow from every point of the object, to be parallel to each other, and to cover the whole reflecting surface DUV F. But to avoid confusion in the figure, we shall only draw two rays of a pencil flowing from each extremity of the object into the great tube, and trace their progress, through all their reflections and refractions, to the eye f, at the end of the small tube tt, which is joined to the great one,

Let us then suppose the object A B to be at such a distance, that the rays C may flow from its lower extremity B, and the rays E from its upper extremity A. Then the rays C falling parallel upon the great mirror at D, will be thence reflected converging, in the direction DG; and by crossing at I in the principal focus of the mirror, they

will form the upper extremity I of the inverted image 1K, similar to the lower extremity B of the object AB: and passing on to the concave mirror L, (whose focus is at n) they will fall upon it at g, and be thence reflected converging, in the direction g N, because is longer than gn; and passing through the hole P in the large mirror, they would meet somewhere about r, and form the lower extremity d of the erect image ad, similar to the lower extremity B of the object A B. But by passing through the plano-convex glass R in their way, they form that extremity of the image at b. In like manner, the rays E which come from the top of the object A B, and fall parallel upon the great mirror at F, are thence reflected converging to its focus, where they form the lower extremity K of the inverted image IK, similar to the upper extremity A of the object A B; and thence passing on to the small mirror L, and falling upon it at h, they are thence reflected in the converging state hO; and going on through the hole P of the great mirror, they will meet somewhere about q, and form there the upper extremity a of the erect image a d, similar to the upper extremity A of the object A B: but by passing through the convex glass R in their way, they meet and cross sooner, as at a, where that point of the erect image is formed. The like being understood of all those rays which flow from the intermediate points of the object, between A and B, and enter the tube at TT; all the intermediate points of the image between a and b will be formed and the rays passing on from the image through the eye-glass S, and through a small hole e in the end of the lesser tube tt, they enter the eye f, which sees the image ad (by means of the eye-glass) under the large angle ced, and magnified in length, under that angle from c to d.

In the best reflecting telescopes, the focus of the small mirror is never coincident with the focus m of the great one, where the first image IK is formed, but a little beyond it (with respect to the eye) as at n: the consequence of which is, that the rays of the pencils will not be parallel after reflection from the small mirror, but converge so as to meet in points about qer; where they will form a larger upright image than a d, if the glass R was not in their way: and this image might be viewed by means of a single eye-glass properly placed between the image and the eye: but then the field of view would be less, and consequently not so pleasant, for which reason, the glass R is still retained, to enlarge the scope or area of the field.

To find the magnifying power of this telescope, multiply the focal distance of the great mirror by the distance of the small mirror from the image next the eye, and multiply the focal distance of the small mirror by the focal distance of the eye-glass: then, divide the product of the former multiplication by the product of the latter, and the quotient will express the magnifying power.

One great advantage of the reflecting telescope is, that it will admit of an eye-glass of a much shorter focal distance than a refracting telescope will; and, consequently, it will magnify so much the more : for the rays are not coloured by reflection from a concave mirror, if it be ground to a true figure, as they are by passing through a convexglass, let it be ground ever so true.

Astronomical Quadrant.

THE following figure represents the portable Astronomical Quadrant mounted on an axis and pedestal.

H

E

The axis allows us to place the instrument in any vertical position.. and the pedestal, moveable in the axis of the circle EF, at the bottom, permits us to place it in the direction of any azimuth, or towards any point of the compass.

The limb A B is graduated into degrees and halves, numbered from A.

Upon the radius C B is fixed a telescope, through which we view any remote celestial or other object. This limb is elevated or depressed by a rack and pinion.

The horizontal circle is graduated into quadrants of 90°, and these again into half degrees, and so on.

m

The point Zero on the quadrant corresponds with Zero on the nonius G.

The quadrant united to the pedestal moves round the limb by means of rack-work and a pinion, F, and shews the position of the place of the quadrant, and consequently of an object.

The rationale of this instrument is very obvious; for by means of it we can measure the angular distance of any celestial body in the zenith, or at any altitude; or of any terrestrial object from any other object in the horizon,

The former is effected by means of the graduated quadrant A B; and the graduated circle EF, accomplishes the latter process. Together, these graduations enable us to determine both the altitude and azimuth of a celestial object, since by the compass we can place the instrument due north and south.

On Observing with the Telescope.

THE apparent magnitude of an object, viewed through a telescope, may be measured, with great accuracy, by a scale or by two wires, introduced at the place of the last image, reducing afterwards the angle thus ascertained according to the magnifying power. Care must, however, be taken to avoid as much as possible the distortion which usually accompanies any curvature of the image; and the wires, one of which is sometimes made moveable, by means of a micrometer screw, must be sufficiently illuminated to be distinctly visible. Sometimes a scale is introduced, which, from the apparent magnitude of a known object, such as that of a man of ordinary height, or of a portion of a wall built with bricks of the usual size, enables us at once to read off its actual distance, which is expressed on the scale in hundreds of yards. The angular magnitude of an object, seen through a telescope, may also be found, by viewing at the same time, with the other eye, either a scale, or any other object of known dimensions, placed at a given distance: the lucid disc micrometer of Dr. Herschel is employed in this manner for judging of the magnitude of the celestial bodies. The divided object glass micrometer affords another mode of measurement: the object glass being divided into two semicircular portions, one of which slides on the other; each portion acts as a separate lens, and two images of every part of the object being formed, the angular distance of any two points is determined by bringing their images together, and measuring the displacement of the moveable portion of the object glass, which is required for procuring the coincidence. Sometimes, also, a similar purpose isanswered by inserting a divided glass in the eye piece, which is nearly on the same principle, and which seems to be somewhat less liable to error. In a reflecting telescope of Cassegrain's construction, Mr. Ramsden has also produced the same effect by dividing the convex speculum, and causing a part of it to turn round an axis.

All these arrangements particularly deserve the attention

of those who are employed in practical astronomy and in geography, since the advancement of these sciences much depends on the accuracy of the telescopic and microscopic measures, which are performed by means of optical instruments.

To Find the Rate of Time-keepers.

IF a Time-keeper could be so constructed as to go uniformly in every season and climate, it would obviate all difficulty respecting the longitude: and though perfect regularity of motion cannot be hoped for in any mechanical contrivance, the want may be in some measure supplied, particularly in short voyages, by ascertaining the rate of a time-keeper; that is, by finding what it gains or loses daily, during a series of observations.

The most general method, as well as the most accurate, of finding the rate of a time-keeper, is by a transit instrument, the use of which will be shewn in the next article.

In order to ascertain the mean daily rate of a watch, it should be some days under trial, and the operation is performed either by taking an arithmetical mean between the two extreme daily variations, or between them all together; the latter method seems the most correct: but if the watch should materially alter its rate of going while under trial, all the observations made before the alterations took place must be rejected, and only those retained which were made afterwards. The time generally allowed for ascertaining the mean rate of a watch is about a month.

But some astronomers think that a rate deduced from a long series of observations is best; while others prefer the latest rate, taken from a short trial. On such a nice point as this, little can be said with certainty. If, however, the rate is found to be uniform, and the watch be well adjusted for heat and cold, a few days may be sufficient to determine its mean rate; but if this be not the case, several months may be necessary.

With respect to the adjustment for climate, as well as the various precautions necessary in the management of a time-keeper, the maker's directions should be carefully followed.

Of the Transit Instrument.

A TRANSIT Instrument is a telescope placed in the meridian in order to observe the times that the heavenly bodies this great pass circle; and thus (by the help of a sidereal clock) to find their right ascensions; and also, to determine the errors and rates of chronometers.

Across the middle of the telescope is fixed an axis, at right angles to it, the ends of which are tapered into pivots, which turn in notches