Those who are old enough to remember the Regent's Park before there were any houses northward of the New Road, may recollect that among the first buildings erected, on what is now called Park Square, was a strange-looking, partly semi-circular erection, provided with ample lighting space, which attracted great attention during its progress, and was the cause of much speculation as to its probable purpose. That building was intended for the exhibition of the Diorama.

M. Daguerre, the inventor of the Daguerreotype, had, in conjunction with M. Bouton, a short time previously opened a similar exhibition in Paris, where the beauty of the paintings, aided by the extraordinary effects of newly contrived dispositions of the light, had excited a great sensation. The Diorama was opened in London on the 6th of October, 1823, and for a long time it was equally popular in this metropolis.

The visitors, after passing through a gloomy anteroom, were ushered into a circular chamber, apparently quite dark. One or two small shrouded lamps placed on the floor served dimly to light the way to a few descending steps, and the voice of an invisible guide gave directions to walk forward. The eye soon became sufficiently accustomed to the darkness to distinguish the objects around, and to perceive that there were several persons seated on benches opposite an open space, resembling a large window. Through the window was seen the interior of a cathedral, undergoing partial repair, with the figures of two or three workmen resting from their labour. The pillars, the arches, the stone floor and steps, stained with damp, and the planks of wood strewn on the ground, all seemed to stand out in bold relief, so solidly as not to admit a doubt of their substantiality, whilst the floor extended to the distant pillars, temptingly inviting the tread of exploring footsteps. Few could be persuaded that what they saw was a mere painting on a flat surface. This impression was strengthened by perceiving the light and shadows change, as if clouds were passing over the sun, the rays of which occasionally shone through the painted windows, casting coloured shadows on the floor. Then shortly the brightness would disappear, and the former gloom again obscure the objects that had been momentarily illuminated. The illusion was rendered more perfect by the excellence of the painting, and by the sensitive condition of the eye in the darkness of the surrounding chamber. Whilst gazing in wrapt admiration at the architectural beauties of the cathedral, the spectator's attention was disturbed by sounds underground. He became conscious that the scene before him was slowly moving away, and he obtained a glimpse of another and very different prospect, which gradually advanced until it was completely developed, and the cathedral had disappeared. What he now saw was a valley, surrounded by high mountains capped with snow. This mountain valley seemed scarcely less real than the arched roof and columns of the cathedral, whilst a foaming cascade, dashing down the rocks, and the sound of rushing waters, added to the illusion. After looking for some time at this beautiful valley, the clouds were seen to gather on the mountain tops, and a storm impended. A gleam of sun-light, still resting on the edge of the clouds, exhibited a strange contrast between the silvery brightness and the dense black vapour that shrouded the hills, and could almost be felt. It was but a passing thunderstorm. Presently the dark clouds rose from the valley, and dispersed; the sun again shone on cottage, vineyard, and mountain, charming the spectator as much by the beauty of the scene as he was astonished by the wonderful change.

Such was the Diorama as it was first exhibited in London to admiring crowds. In subsequent years greater changes were made in the variations of light and shade; and by the introduction of mechanical contrivances, with more or less success, the magical effects were increased, without, however, adding to the apparent reality of the objects. A church or cathedral was always the subject of one view, and sometimes of both. The interior of an empty church would be shown by evening twilight. The shades of evening gradually darkened into the obscurity of night, and then the glimmer of candles would be seen spreading more and more widely, until the church was lighted up, and it was occupied by a crowded congregation at midnight mass. Some views represented the exterior of a ruin or of a cathedral after sunset, and as night advanced, the stars twinkled in the blue sky, and the moon rose and threw its silvery light on water, buildings, and clouds, contrasting in some cases with the red glare of lamps from the windows of houses and shops. The disc of the moon exactly resembled that of the real luminary, and all around being so dark, the rays from its surface cast shadows of intervening objects. In one picture a still more astonishing appearance was produced, by the change of the interior of a beautifully painted and decorated church into a mass of charred ruins.

The means principally adopted for the production of these magical changes in a painting on a flat surface, and for giving such seeming reality to the objects represented, were for some time kept secret; nor do we think they are even yet much known. As in many other clever inventions, the effects are produced in a very simple manner. The picture is painted on both sides of a transparent screen, and the change of scene is occasioned almost entirely by exhibiting the picture at one time by reflected light, from the surface nearest the spectator, and afterwards by transmitted light, after excluding the light from the front.

Let us take for illustration the interior of a church, at first seen empty, and afterwards filled with people, and illuminated by candles. The empty church is painted on the front on fine canvas or silk, in transparent colours, and at the back are the figures and candles, and other objects intended to appear with them. The arrangements for illuminating the picture are so contrived, that the light may be thrown entirely on the front or on the back, or partly on both. When the light is on the front, the empty church only is visible. It is then gradually darkened, and the back of the picture is illuminated, by which means the figures and candles are seen; and the form of the building being preserved, the same church, which was before empty, becomes occupied by a crowded congregation.

It may be mentioned, as an illustration of the perfect illusion of the Diorama, that a lady who on one occasion accompanied the author to the exhibition, was so fully convinced that the church represented was real, that she asked to be conducted down the steps to walk in the building.

The effect of changing the direction of the light may be readily perceived by making a drawing on both sides of a sheet of paper, as shown in the annexed engraving. The side backing this page represents the interior of St. Paul's Cathedral when empty, and on the back several figures are drawn. Those figures are invisible until the leaf is held up against the light, and when the drawing is seen as a transparency, the objects on the back, as well as those in front, come into view, and the building appears to be occupied.

Any one who has a taste for drawing, and a little ingenuity, may thus produce many pleasing and astonishing effects. It will be desirable to procure, in the first instance, a box, so contrived that it will hold the painting, and afford the means of throwing the light on the front or on the back at pleasure. The diagram shows the form of such a box. The letters a, b, c, d mark the outside; the aperture, at c d, being enlarged to permit several persons to look into it at the same time. The box may be of any required dimensions, to suit the size of the drawing, which is to be fitted into a groove at a b, and the interior must be blackened. The lid, e, when open, as in the diagram, admits the light to the front of the picture, the back being covered with an opaque screen. As the lid is closed, the picture becomes darkened, and by the gradual removal of the screen at the same time, it is changed into a transparency. This portable Diorama can be most conveniently shown by lamplight, the flame of an argand lamp, the wick of which can be heightened and lowered, being best adapted for the purpose. The effect by daylight is, however, superior, but the room must then be darkened, and the admission of light confined to the picture.

The moving water, and the motion of smoke and clouds, which were frequently introduced in the Diorama, were mechanical additions, the effects being produced by giving motion to bodies behind, the forms of which were seen by transmitted light. The introduction of such mechanical aids, however, detract from the artistic character of the Diorama, the principal merit of which consists in exhibiting the changes occasioned by variations in the mode of throwing the light on the two-faced picture.

It is to be regretted that exhibitions of a larger and more showy kind should have superseded the Diorama in public estimation; and that, from the want of support, their charming and marvellous pictorial representations, which formed, in days gone by, one of the principal "sights" of London, should be now closed.