THE OPTICIAN.

The Optician makes telescopes, microscopes, spectacles, opera-glasses, reading glasses, &c. &c.

The history of this important art will, in effect, be an account of the art itself, which we shall endeavour to give in as concise and
perspicuous a manner as we can, consistent with the design of this work.

Although the ancients made few optical experiments, they nevertheless knew that when ight passed through media, of different! densities, it did not move in a straight line, but was bent or refracted out of its original direction. This was probably suggested to them by the appearance of a straight, rod partly immersed in water; and accordingly we find many questions concerning this, and other optical appearances in the works of Aristotle. It appears also from Pliny, and Lactantius, that burning glasses were known to the ancients.

Archimedes is said to have written a treatise on the appearance of a ring or circle under water, and therefore could not have been
ignorant of the common phenomena of refraction.

The ancients, however, were not only acquainted with these more ordinary appearances, bat also with the production of colours, by refraction. Seneca says, that if the light of the sun shines through an angular piece of glass, it will shew all the colours of the rainbow. The first treatise of any consequence, on the subject of optics, was written by Ptolemy ; this treatise is now lost, but from the accounts of others, we find that he treated of astronomical refractions.

The nature of refraction was afterwards considered by Alhazen, an Arabian writer ; and his observations were afterwards confirmed by Vitellio, Tycho Brahe, and others.

In the writings of Roger Bacon, in the thirteenth century, we find the first distinct account of the magnifying power of glasses, and
it is not improbable, that what he wrote upon this subjeet, gave rise to the useful invention of Spectacles. From this time to that of the revival of learning in Europe, we have no treatise on optics. One of the first who distinguished himself in this way, was Maurolycus, teacher of mathematics at Messina, in 1578. Baptista Porta, who died in 1515, was the inventor of the camera obscura, which throws more light on these interesting subjects. From this period, the writers on optics have been numerous and important, amongst whom Sir Isaac Newton ranks as one of the most eminent.

Glass globes, and specula, seem to have been the only optical instruments known to the ancients, Alhazen gave the first hint of the
invention of spectacles. From the writings of this author, together with the observations of Roger Bacon, it is not improbable that some monks gradually hit upon the construction of spectacles. It is certain that spectacles were well known in the 13th century, and not long before. It is said, that Alexander Spina, a native of Pisa, who died in 1313, happened to see a pair of spectacles in the hands of a person who would not explain them to him, and that he succeeded in making a pair for himself, and immediately made the construction public. It is also inscribed on the tomb of Salvinus Armatus, a nobleman of Florence, who died in 1317, tbat he was the inventor of spectacles.

But although convex and concave lenses were sufficiently common, yet no attempt was made to combine them into a telescope, till
the end of the sixteenth century. We are informed, that as James Metius was amusing himself with mirrors and burning glasses, he
thought of looking through two lenses at a time ; and that happening to take one that was convex, and another that was concave,
and happening also to hit upon a pretty good adjustment of them, be found, that by looking through them, distant objects appeared very large and distinct In fact, without knowing it, he had made a Telescope,

But the honour of having exhibited this arrangement of glasses in a tube, appears due to Jansen, a spectacle maker, of Middleburgh, in 1590. Jansen, directing bis telescope to celestial objects, distinctly viewed spots on the surface of the moon, and discovered many new stars.

Galileo having made many improvements in the telescope has by some been considered as the inventor, but he himself acknowledges that he first heard of the instrument from a German. The first telescope which Galileo constructed, magnified only three times; but soon after he made another, which magnified eighteen times ; and afterwards, with great trouble and expense, he constructed one which magnified thirty three times, and with this he discovered the satellites of Jupiter, and the spots on the sun.

The honour of explaining the principles of the telescope is due to Kepler.

The principal effects of telescopes depend upon these simple principles, viz. that objects appear larger in proportion to the angles which they subtend at the eye ; and that the effect is the same whether the pencils of rays by which objects are visible to us, come directly from the objects themselves, or from any place nearer to the eye where they may have been conveyed, so as to form an image of the object; because they issue again from those points when there is no real substance in certain directions, in the same manner as they did from the corresponding points in the objects themselves.

In fact, therefore, all that is effected by a telescope, is first to make such an image of a distant object by means of a lens or mirror 5
and then to give to the eye some assistance for viewing that image as near as possible: so that the angle which it shall subtend at the
eye, may be very large compared, with the angle which the object itself would svibiend in the same situation. This is done by means of any eye-glass which so refracts the pencil of rays, that they may afterwards be brought to their several foci by the humours of the eye. But if the eye was so formed as to be able to see the image with sufficient distinctness at the same distance without an eye-glass, it would appear to him as much magnified as it does to another person who makes use of a glass for that purpose, though he would not in all cases have so large a field of view.

Such is the telescope which was first discovered and used by philosophers. The great inconvenience attending it is, that the field of
view is exceedingly small. This inconvenience increases with the magnifying power of the telescope, so that it is a matter of surprise
how, with such an instrument, Galileo and others could have made such discoveries. No other telescope, however, than this, was so much as thought of for many years after the discovery.

It is to the celebrated Kepler, that we are indebted for what we now call the astronomical telescope. The principles of this instrment are explained, and the advantages of it are clearly pointed out by this philosopher in his Catoptrics ; but what is very surprising, he never actually reduced his theory to practice. :

The first person who made an instrument of Kepler's construction was Scheiner, who has given a descriptioii of it in his Rosa Ursina, published in 1630. If, says he, you insert two similar lenses in a tube, and place your eye at a convenient distance, you will see all terrestrial objects inverted, indeed, but magnified and very distinct, with a considerable extent of view. He afterwards subjoins
an account of a telescope of a different construction with two convex eye-glasses, which again reverses the images, and makes them appear in their natural position. This construction, however, answered the end very imperfectly, and Rheits soon after discovered a better construction, using three eye-glasses instead of two.

But these improvements and many others since made, have diminished in value by the discovery of the reflecting telescope ; for a refracting telescope even of 1,000 feet focus, supposing it possible to be made use of, could not be made to magnify with distinctness more than 1,000 times, whereas, a reflecting telescope not exceeding nine or ten feet, will magnify 1200 times.

Mr. James Gregory of Aberdeen, was the first inventor of the reflecting telescope, but his construction is quite different from Sir
Isaac Newton's, and not nearly so advantageous. But in constructing reflecting telescopes of extraordinary magnifying powers, Sir William Herschel has displayed skill and ingenuity surpassing all his predecessors in this department of mechanics. He has made them from 7, 10, 20, to even 40 feet in length, and with instruments of these dimensions he is now employed in making discoveries in astronomy.

To describe these instruments would far exceed the limits to which we are confined ; but we may mention that the concave face of the metallic mirror of Sir William's largest telescope, which is fixed at the bottom of a forty-feet tube of iron, is forty-eight inches of
polished surface in diameter. The thickness, which is equal in every part of it, is about three inches and a half, and its weight, when it came from the cast, was 2,118 pounds, of which it must have lost a small quantity in the polishing. The metal is an amalgam supposed to be composed of 32 parts of copper, 15 of tin, 1 of brass, 1 of silver, and 1 of arsenic : for Sir W. Herschel has not made the composition public ; but Mr. Edwards, an intimate friend of his, after repeated trials, found this proportion the best for receiving a fine polish,and producing the most perfect reflection.

This instrument, with proper eye-glasses, magnifies above 6,000 times, and is the largest which has ever been made.

The achromatic telescope was the invention of Mr. Peter Dollond.

The micrometer is an instrument which is used with a telescope for the purpose of measuring small angles, and by the help of which
the apparent magnitudes of objects viewed through a telescope or microscope, are measured with great correctness.

The microscope is composed of lenses or mirrors, by means of which small objects are made to appear larger than they really are to the naked eye. Microscopes are distinguished into simple, compound, and double. Simple microscopes consist of a single lens or spherule. The compound microscope consists of several lenses duly combined. As optics have each improved, other varieties have been contrived in this instrument : hence, we have reflecting microscopes, water microscopes, botanical microscopes, solar microscopes, &c.

The kaleidoscope is an instrument which has lately obtained great celebrity on account of the very amusing and new forms which, by turning it round, it constantly presents to the eye. Dr. Brewster of Edinburgh has obtained a patent for the invention, an account of which may be seen in the Monthly Magazine for January, 1818. It is asserted, however, that the discovery is not a new one for that a person named Bradley, a gardener at Hampton Court, mentions such an instrument in a work published by him more than one hundred years ago.

The mode in which the kaleidoscope is made Is very simple : take a hollow tube of any dimensions, and of auy length, two inches in diameter and twelve long is a convenient size : take two pieces of plate glass about one inch and a half in diameter, and one line in thickness, of a length somewhat shorter than the tube itself, and let them be fixed so that one edge may touch the other, and so as to form fin angle with each other of 22 and one half degrees ; a few bits of cork may be so notched as to keep the pieces of glass in their places: the glasses are to be darkened by black painting, or some other convenient method on the exterior sides. At one end of the tube provide two circular pieces of plain clear glass, exactly the diameter of the tube into which they are to be fitted, Place between these two glasses a quantity ot broken pieces of different coloured glass, the more intense and various the colours the more brilliant will the forms be, and let the pieces of broken glass be so placed as to move freely as the tube is turned round. At the opposite end of the tube let there be a small hole for the sight : the instrument will be complete ; a succession of beautiful forms will then be visible, which, till experienced, would be believed absolutely impossible to be produced by any act or contrivance of man. The uses to which this instrument may be put, both useful and ornamental, it would not be easy to enumerate; it can never cease to be a constant source of amusement and delight.

Telescopes are made of various dimensions, and at a great variety of prices.

Spectacles are also an article, as is well known, in considerable request. Theyare made to suit eyes of different ages and of different capacities of vision. Their prices are various, depending principally upon the style in which they are mounted.

From what has been said, it is evident that an Optician should be conversant with mathematics and mechanics, and many other
branches of science with which optics are connected. He should also know the history of what has been hitherto done in this art, as
well as what is now doing, in order to be able to apply himself to the construction of the various instruments which it is his business to make.