640

Panorama and Cosmorama.

becomes exactly what belongs to the supposed remoteness of the objects, and it also bends the whole beams of light so that the axes of the eyes may be nearly parallel.

870. The effects of the magnitude and distance of the ordinary large panoramic views may, with the assistance of suitable glasses, be obtained from even a very small picture or engraved representation embracing the same field. An enterprising artist might undertake to offer for sale a variety of such views at little cost. A common panorama picture, covering a circular wall of 100 feet in circumference and many feet high, may be reduced, still retaining the same truth of proportions, to appear as an engraving on paper five feet long and eight inches high. With the arts of lithography and photography now so well adapted for producing soft representations of scenery, the expense of such views might be rendered so moderate as to allow of their becoming a common part of library furniture. When we reflect upon the expansion of thought obtained by travelling, and that not a few of the advantages of travelling would follow a familiarity with a good selection of panoramic views, it appears that courses of instruction in geography and history may be more commonly illustrated than now, by this very interesting mode of aiding the conception and memory. This want has been in some measure supplied by photography.

Common paintings and prints may be considered as detached parts of a panoramic representation, showing as much of that general sphere of vision which always surrounds a spectator, as can be seen by the eye kept in one place, and looking through one window or other opening of moderate size. The pleasure from contemplating these, is much increased by using with them a lens or such spectacles as above described.

The stereoscopic landscape views on glass, now become common, surpass all other representations of real scenes in being mathematically and minutely accurate, and in the curious fact, due to the double pictures, that objects behind hidden from one eye by objects in front are visible to the other eye.

871. An interesting kind of representation has been exhibited in London and Paris under the title of Cosmorama (from Greek words signifying sights of the world, because of the great variety of views). Pictures of moderate size are placed outside of what appear ordinary windows in a darkened room, but which are really large convex lenses fitted to correct the errors of appearance which the nearness

of the pictures beyond them would else produce. Then, by the addition of various subordinate contrivances, calculated to aid and heighten the effects, they lead even shrewd judges to suppose the pictures of moderate size behind the glasses to be large elaborate works. To children they appear as magical realizations of natural scenes and objects.

From what has now been said, it appears that for the purpose of representing still-nature, or mere momentary states of moving objects, a picture truly drawn, truly coloured, and which is either large, to correct the divergence of light and convergence of visual axes, or if small, is viewed through a proper lens, would affect the retina almost exactly as the realities. But the desideratum remained of being able to paint motion Now this, too, has been attempted, and in some cases with singular success, chiefly by making the picture transparent, and throwing lights and shadows upon it from behind. In the exhibitions of the Diorama and Cosmorama there have been thus represented with admirable truth, such phenomena as the clear sunlight of a summer's day occasionally interrupted by passing clouds; the gradual rising and disappearing of a mist over a wide landscape; running water, as pouring down in the mighty Falls of Niagara, or the still loftier cascades among precipices of Alpine regions, and even the appalling spectacle of a furious conflagration. One part of the mechanism which produced these effects was a circular frame of canvas turning slowly as a wheel behind the main picture, on which canvas strongly illuminated from the back were painted forms of flame and smoke.

The invention in our day of the stereoscope, and some other discoveries made regarding light and vision, as in photography, have added a new interest to this department of natural philosophy, and may lead to further improvements in the painter's art. Of many objects and subjects, knowledge can be conveyed more quickly and completely by pictorial images than by words, and as things differ so much in their nature, distinct classes of artists have arisen to represent them. There are special painters of flowers and fruit, of birds and of other animals, of landscapes, sea-pieces, and the interiors of buildings, and especially of the human countenance and form, singly in portraits, and conjointly in scenes of human action, as related in history. To attain moderate proficiency in some of these departments, moderate ability suffices, but to reach excellence in others, the highest natural endowments of intellect and feeling, with educational cultivation, are required.

642

Invention of the Telescope.

"When the image formed, as above described, beyond a lens, is viewed in the air by an eye placed still farther beyond in the same direction, the arrangement, according to minor circumstances, constitutes either the TELESCOPE or the MICROSCOPE."

872. The name TELESCOPE (a compour.d Greek term, signifying to see far, as microscope signifies to see what is small), applies to that marvellous instrument of comparatively modern invention, by the use of which the intelligent mind may be said, on the beams of light as a path, to bound widely into space for the purpose of examining more closely the great distant bodies of creation; or, by which it seems able to command distant objects instantly to approach, for the purpose of convenient inspection. The telescope is the instrument by which this is effected. One which merely doubles the apparent diameter shows the moon exactly as she would appear to a person who had ascended towards her from the earth through a distance of 120,000 miles, while one of greater power produces effects correspondingly great. But to examine the heavenly bodies is only one of the many uses of the telescope. The instrument, fixed on the graduated brass circle of the theodolite, enables us to measure angles, which tell the exact distance of one mountain summit from another, even if a river or a wide sea intervene. Again, men have often wished to discover what is passing at a distance on the surface of the earth around them. Thus, by a telescope, the military chief may obtain a close view of approaching friends or foes while they are yet concealed from the naked eye in the blue mist of distance; and similarly, the sea-captain, while persons around him perceive only a small dark speck on the far horizon, discovers that to be a ship of a class and nation at once evident to him, and with the crew of which, by the additional use of signal flags, he is enabled readily to communicate. At midnight, a telescope directed to a distant cathedral tower may watch on the clock-face the motion of the hands marking the unceasing lapse of time. A man placed in the midst of a wide plain, or on a lofty hill-top, or far on the face of a lake, who might suppose himself quite alone and unseen, might yet, through a telescope, be instantly placed under the observation of any one choosing to watch him. The same might happen to a man within the high walls of his own garden, or even within his house, near an open window, if a straight line could pass from him to an observer.

Construction of the Telescope.

643

Now the telescope, with its marvellous powers mentioned, exhibits but a modificaton of the simple case, described in Art. 816, and exemplified in the camera obscura, of an image formed for visual inspection, by the rays of light gathered to a focus beyond a lens. We have here to explain that its powers depend on the two facts, first, of its large lens collecting for the formation of the image (subsequently transferred to the observer's retina) a thousand times or more the quantity of light which the naked pupil admits; and, second, of its forming by this light a large bright image, to which the eye may approach very near, to examine it through a magnifying glass of any power.

873. To understand this fully, we must recall, as explained in Art. 846, that the nature of the bending of light in passing through a lens is such, that all the rays reaching the lens from any point of a visible object in front (as the point A of the cross, A B, fig. 213), and forming what is

called a pencil or cone of light, are collected in a corresponding

Fig. 213.

point, as a, at the focal distance beyond the centre of the lens, so as to meet the central ray of the pencil (here the direct line, a a); and then, because the same happens to the light from every visible point of the object, the collected light from all received on a white screen placed there, produces a beautiful inverted image of the object. In fig. 213, to prevent confusion, the rays from the extreme points, A and B, are alone represented. If no screen be interposed in the place where the rays meet to show this image, the rays, although not seen, are not lost there or disturbed, but merely cross in the air and pass on, diverging again beyond the focal points, or towards c, as they originally did from the several points of the object itself. An eye, therefore, placed beyond c, in the line of the rays, must receive the light from every point of the image, and will see the image in the air as it would see an object situated where the image is. This fact is tested at once by holding a spectacle glass or any lens at a proper distance from the eye between an object and the eye. An inverted image of the object is seen.

874. A telescope, then, is merely a tube, converted into a dark chamber by excluding useless light, and having a large lens, called the object glass, filling its distant end like a window, through which the light from the objects in front enters, to form images towards the

644

Astronomical Telescopes.

other end of the tube, where the eye may conveniently inspect them. The inspection of the image is made through another lens called the eye-piece of the telescope, which is fixed in a small tube made to slide backwards and forwards in the larger, so as to admit of the focal distar.ces being adjusted to the power of different eyes. The 1.

Fig. 214.

accompanying figure (fig. 214), A in which, for the sake of simpli

city, the exter

nal tube of the telescope does not appear, shows the progress of the light from points of the object, A, through the object-glass, L, to form an image at b a, and afterwards to be bent by the eye-piece, D, so as to enter the pupil of the eye at E, where the rays cross, to form the last magnified image on the retina.

In the simple telescope, having only two lenses, as above represented, and called the astronomical telescope, or sometimes the night-glass, when used by mariners at night, the image is inverted. This circumstance is of no importance in viewing the heavenly bodies, which are round, from whatever side seen. To fit the telescope, however, for viewing terrestrial objects, it is necessary to place in the tube another simple or compound lens, D, which shall turn the inverted image formed by the great object-glass into an image which is upright. There is considerable loss of light where it is necessary to multiply lenses.

875. In order to determine the magnifying power of a telescope, or how much larger an object will appear when viewed through a telescope with an object-glass of three feet focus, than when viewed by the naked eye, we must recollect that the image is formed in the focus of the object-glass, or at ba, in fig. 214, and subtends from the centre of that glass or lens the same visual angle as the object itself, viewed from the same point (a fact explained in Art. 820), and to an eye placed at the lens would appear of the same size as the object; but if the eye be brought nearer to the image than the centre of the object-glass, L, the image will appear in proportion just so much taller and broader, and thus, as compared with the object, may be called so much magnified. Now, as the naked eye cannot see distinctly an object nearer to it than at about six inches distance, because of the great divergence of light from nearer radiant points, the telescope in question, without an eye-glass, would allow the eye to come only six times nearer to the image than wher.