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tion. (A good idea of such a precipitation is had by observing the particles of water condensed from transparent vapor, in unusually high thunder-heads, where the action is in some respects similar.) Between these ascending columns are seen descending masses of cooler vapors, rendered dark and smoky by relatively cool and opaque particles of carbon, all or most of the other elements being still maintained by the excessively high temperature in the condition of transparent vapor. In the immediate region, however, where the cyclone is raging, these bright ascending columns are drawn out horizontally by the inrushing metallic winds (which often reach a velocity of a thousand miles per hour) into long filaments, pointing in general toward the center of the disturbance, which is always occupied by a huge black cloud of smoke (frequently twenty thousand miles in diameter) rapidly settling back into the interior of the sun. Over and across this great central black cloud are often driven long arms of the shining carbon-clouds, which, when the cyclonic action is very strong, bend round into slowly changing spiral forms, very suggestive of intense action. A striking illusion, invariably connected with this sight, is that the observer seems to be viewing it from a position quite near the scene of the disturbance, whose minute and complicated details are seen with exquisite distinctness.

After witnessing such a spectacle, the observer must have felt great admiration for the men who have devised and successfully constructed an instrument capable of showing in action such enormously energetic forces, the very existence of which would otherwise hardly have been conceived.

But, although the refracting telescope has now been brought to such exquisite perfection, the first ones were exceedingly crude, and it is interesting to trace the gradual development of the telescope from a simple pair of spectacle-glasses, suitably placed one behind the other, into the great refractors of Washington, Vienna, and Pulkowa, which are monuments of optical and mechanical ingenuity.

Spectacles were invented about the year 1300, but it was not until 1608 that a Dutch spectacle-maker, as a pretty experiment, combined two such lenses in a way that made distant objects look nearer. A rumor of this invention reached Galileo, at Venice, in 1609, and interested him so much that, before he had even seen one of them, he reasoned the problem out for himself, and in a few days produced a telescope which made distant objects appear to be only one third as far away as they actually were, by cementing a suitable spectacle-glass in each end of a lead organ-pipe. With this instrument the astonished senators of Venice derived great amusement in spying out ships at sea from the top of the great bell-tower.

So industriously did Galileo follow up his first achievement, that soon he had constructed more than one hundred telescopes of various sizes, one of which made objects look eight times nearer; and, finally,

with great exertion and expense, completed one magnifying thirty diameters, which we now know to be the greatest power possible with the form of lenses that he used, viz., a double-convex lens for the object-glass and a double-concave lens for the eye-piece.

With such crude instruments as these, Galileo made his wellknown discoveries, which, coming just when they did, proved of great importance in giving an additional impulse to the then rapidly awakening intellect of Europe.

Soon after the death of Galileo the telescope was further perfected by Huygens, who, in the first place, invented the form of eye-piece which still bears his name, and gives a large, flat field with very sharp definition. Many variations of form, but no improvement in the seeing quality of telescopic eye-pieces, have since been made, so that from this time all improvements in the telescope have been necessarily confined to the object-glass.

Huygens next enlarged the single-lens object-glass to its greatest possible power. His largest telescope had an object-glass five inches in diameter, and a focal length of one hundred and twenty feet; this enormous focal length being absolutely necessary to reduce the blurring effect of the prismatically colored fringes, as well as spherical aberration, to such moderate limits that a magnifying power of upward of two hundred diameters could be employed.

To have watched Huygens at work with this telescope must have been an amusing sight. Its great length precluded the use of a tube, and therefore an assistant was obliged to slide the object-glass up and down a vertical pole, one hundred feet high, by a cord, while Huygens pointed the eye-piece at the object-glass by sighting along a string connecting the two, meanwhile steadying himself by resting his elbows on a two-legged wooden horse. A more difficult and unsatisfactory contrivance to use can hardly be imagined, yet, with this telescope, in 1655, he discovered the rings of Saturn, and one of its satellites.

Newton, about this time, hastily concluded, from experiments of his own, that refraction without prismatic color was out of the question, and that the refracting telescope was incapable of further improvement; he therefore abandoned the study of the refracting telescope, and turned his attention to the construction of reflectors, and thus narrowly escaped making that most important discovery-the achromatic object-glass-which, only two years after his death, actually was made by Dollond, who, in 1757, constructed one two and a half inches in diameter, corrected both for prismatic color and spherical aberration.

From that day the power of the refracting telescope rapidly increased, and up to the present moment has only been limited by the ability of the glass-makers to furnish large pieces of optically perfect glass.

The completely equipped telescope, with its object-glass and mounting, aside from being a triumph of the highest optical and mechanical skill, is certainly the noblest instrument that man has yet constructed, and it is difficult to decide which is the most sublime and elevating to contemplate-the universe, which the telescope enables us to see, measure, weigh, and, combined with the spectroscope, to analyze; or the exquisite mechanism, by means of which light is first originated, then propagated, and finally refracted to an image on the retina of the eye.

We shall, in what follows, briefly consider the latter subject, which will enable us to understand the natural laws that render possible the remarkable degree of perfection and power to which the refracting telescope has been carried, and which also fix a limit to its indefinite improvement.

Light is the sensation produced on the retina of the eye by some force, usually emanating from a luminous body, but not always, for the same sensation may also be produced by a current of electricity, or by a quick blow on the ball of the eye.

At the first glance this force, which has such a remarkable effect upon the retina of the eye, seems to be a rather difficult thing to interrogate in a way to make it divulge something of its true nature; and so it really proved, for even Sir Isaac Newton, with all the facts known in his day, and with the splendid work of Huygens on the undulatory theory of light before him, failed to satisfy himself on that point; and, in fact, it required the combined work of Young, Fresnel, and many others, extending over a period of two hundred years, to demonstrate beyond question that the one and only explanation admissible is the undulatory theory first propounded by Huygens.

At the present time, however, it is possible to state with certainty a great deal regarding the true character of this force called light.

A revolving mirror, properly combined with one that is stationary, shows that light travels between them through a vacuum with the almost inconceivable velocity of 186,000 miles per second; while other . experiments prove that this is also the velocity of light through space from star to star.

The diverse and curious phenomena called diffraction, interference, and dispersion, show that light consists of vibrations or waves in some transmitting medium, and therefore that this medium must fill the whole visible universe.

The phenomenon called polarization of light shows that the motion of each particle of the medium as it vibrates is at right angles to the direction in which the waves are propagated, and, strange to say, that the medium transmitting them has the properties of a solid substance, and not those of a fluid, such as a liquid or a gas. A good idea of this kind of a wave is had by observing the wave propagated along a tightly stretched telegraph-wire when it is struck a smart blow with a

stick. Although many of the properties of the light-waves are also common to all forms of wave-motion, yet others are distinctively due to the waves being of this particular kind. This form of wave, therefore, is to be carefully distinguished from that propagated in a fluid, where there is always a forward and backward motion to the particles. For example, in the familiar case of waves on the surface of water, the particles of water move in circular paths as the waves pass by—that is, each particle moves forward and back exactly as far as it moves up and down. Also in the case of sound-waves, which are waves propagated through a gas, the particles of the air move only forward and back along the line in which the sound-waves are advancing.

The diffraction grating shows that the waves which produce the sensation of light are very minute, and are of every possible length, between the limits of 32,000 to the inch to 64,000 to the inch, measured from crest to crest. This is only one fifth of the total range of wave-lengths that have been measured radiating from the sun, but only those longer than 3 of an inch, or shorter than 400 of an inch, ordinarily reach the retina to produce the sensation of light. The diffraction grating also shows that the color of light is due directly to the length of the waves, the longest producing the sensation of red light, the shortest of violet, while ranged in between come the various shades of orange, yellow, green, and blue.

Diagram 1 will perhaps give a better idea of the true size and number of the light-waves than is possible from a mere statement of their length and velocity in figures. It represents in section, magnified five hundred diameters, a series of crests of the longest waves that affect the eye as light, passing through a hole in writing-paper, pricked by an ordinary No. 12 sewing-needle, measuring one seventy-fifth of an inch in diameter. It will be noticed that, although the magnified diameter of the hole appears nearly seven inches across, yet the equally magnified crests of the light-waves are still only just far enough apart to be distinctly separated by the eye. On this scale the pupil of the eye would appear nine feet across, and a very good idea of the number of these particular waves, which enter the eye in a continuous stream whenever it receives the light of a distant object, can be had by considering that, if every one of these light-waves passing through the needle-hole in a single second had been represented on the diagram, one behind the other, they would have formed a band extending in the direction of the arrow to a distance of nearly 100,000,000 miles, and to have shown them all on the diagram would have necessitated the paper being long enough to reach from the earth to beyond the sun!

Having once established the fact that the sensation of light is caused by waves originated in the sun and stars, falling upon and irritating the retina of the eye, it of course follows that space must be

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