the box X, and having removed the mirror H, as well as the board F, and the rules D E, incline the moveable mirror L L, till it make with the horizon an angle of nearly 45°, if you intend to represent objects at a considerable distance, and which form a perpendicular landscape. When this is done, all those objects which transmit rays to the mirror LL, so as to be reflected on the convex glass, will appear painted on the paper frame: the point where the images are most distinct may be found, if the tube which contains the lens be lowered or raised, by screwing it up or down.

By these means any landscape, view of a city, &c, may be exhibited with the greatest precision.

II. To represent objects in such a manner, as to make that which is on the right appear on the left, and vice versa.

The box X being in the situation represented in the figure, open the door B, and having placed the mirror H in the slit, and in the situation already mentioned N° 5, raise the mirror L L till it make with the horizon an angle of 224 degrees; if the fore part of the machine be then turned towards the objects to be represented, which we here suppose to be at a considerable distance, they will be seen painted on the paper, but transposed from right to left.

It may sometimes be useful to make a drawing where the objects are transposed in this manner; for example, in the case when it is intended to be engraved; for as the impression of the plate will transpose the figures from right to left, they will then appear in their natural position.

III. To represent in succession all the objects in the neighbourhood, and quite around the machine.

Place the mirror H in a vertical position, as seen in the figure, and incline the mirror L at an angle of 45 degrees; if the former be then turned round vertically, the lateral objects will be seen painted in succession on the paper, in a very pleasant manner.

It must here be observed, that it will be necessary to cover the mirror H with a kind of box made of pasteboard, open towards the objects, and also towards the aperture N of the box X; for if the mirror H were left entirely exposed, it would reflect on the mirror L a great many lateral rays, which would considerably weaken the effect.

IV. To represent the image of paintings or prints.

Affix the painting or print to the side of the board F, which is next to the mirror L, and in such a manner that it may be illuminated by the sun. But as the object in this case will be at a very small distance, the tube must be furnished with a glass, having its focal distance nearly equal to half the height of the machine above the paper if the distance of the painting from the glass be then equal to that of the glass from the paper, the figures of the painting will be represented on the paper exactly of the same size.

The point at which the figures have the greatest distinctness, may be found, by moving backwards or forwards the board F, till the representation be very distinct.

Some attention is necessary in regard to the aperture of the convex glass.

In the first place, the same apérture may in general be given to the glass as to a telescope of the same length.

Secondly, this aperture must be diminished when the objects are very much illuminated; and vice

versa.

Thirdly, as the traits appear more distinct when the aperture is small, than when it is large, if you intend to delineate the objects, it will be necessary to give to the glass as small an aperture as possible; but taking care not to extenuate the light it will therefore be proper to have different circles of cop per or of blackened pasteboard to be employed for altering the size of the aperture, according to cir

- cumstances.

REMARK.

On the top of the Royal Observatory, at Green. wich, is an excellent camera obscura, capable of containing five or six persons, all viewing the exhibition together. All the motions of the glasses are easily performed by one of the persons within, by means of attached rods; and the images are thrown on a large and smooth concave table, cast of plaster of Paris, and moveable up and down so as to suit the distances of the objects.

PROBLEM III.

To explain the nature of vision, and its principal phe

nomena.

BEFORE We explain in what manner objects are perceived, it will be necessary to begin with a de.

scription of the wonderful organ destined for that purpose.

The eye is a hollow globe, formed of three membranes, which contain humours of different densities, and which produces in regard to external objects the same effect as the camera obscura. The first or outermost of these membranes, called the sclerotica, is only a prolongation of that which lines the inside of the eye-lids. The second, called the choroides, is a prolongation of the membrane which covers the optic nerve, as well as all the other nerves. And the third, which lines the inside of the eye, is an expansion of the optic nerve itself: it is this membrane, entirely nervous, which is the organ of vision; for, notwithstanding the experiments in consequence of which this function has been ascribed to the choroides, we cannot look for sensation any where else than in the nerves and nervous parts.

In the front of the eye the sclerotica changes its nature, and assumes a more convex form than the ball of the eye, forming here what is called the transparent cornea. The choroides, by being prolonged below the cornea, must necessarily leave a small vacuity, which forms the anterior receptacle of the aqueous humour. This prolongation of the choroides terminates at a circular aperture well known under the name of the pupil. The coloured part which surrounds this aperture is called the iris or uvea; it is susceptible of dilatation and contraction, so that when exposed to a strong light the aperture of the pupil contracts, and in a dark place it dilates.

This aperture of the pupil is similar to that of the camera obscura. Behind it is suspended, by a circular ligament, a transparent body of a certain con

sistence, and having the form of a lens: it is called the crystalline humour, and in this natural camera obscura performs the same office as the glass in the artificial one.

By this description it may be seen, that between the cornea and the crystalline humour there is a sort of chamber, divided into nearly two equal parts, and another between the crystalline humour and the retina. The first is filled with a transparent humour similar to water, on which account it has been called the aqueous humour. The second chamber is filled with a humour of the same consistence almost as the white of an egg: it is known by the name of the vitreous humour. All these parts may be seen represented plate III, fig. 9; where a is the sclerotica, b the cornea, c the choroides, d the retina, e the aperture of the pupil, ff the uvea, b the crystalline humour, ii the aqueous, k k the vitreous, and the optic nerve.

As it is evident, from the above description, that the eye is a camera obscura, but more complex than the artificial one before described, it may readily be conceived that the images of the external objects will be painted in an inverted situation, on the retina, at the bottom of it; and these images, by affecting the nervous membrane, excite in the mind the perception of light, colours, and figures. If the image be distinct and lively, the impression received by the mind is the same; but if it be confused and obscure, the perception is confused and obscure also: this is sufficiently proved by experiment. That such images really exist, may be easily shewn by employing the eye of any animal, such as that of a sheep or bullock; for if the back