A solar microscope is constructed in the following manner : In the inside of a tube is placed a convex lens, A B (fig. 22) ; and at a distance a little greater than its focal length, but less than double of it, is fixed some transparent coloured object, Q P, whose image will be painted much enlarged at the focus conjugate to the place of the object. A broad lens C D, is placed before the object to col lect the solar rays, for the purpose of illuminating it more strongly, and con sequently making the image more distinct and vivid. On the same principle is constructed the AL1G1C LANTERN, which see.
The camera obscura is an instrument used to facilitate the delineation of pros pects. It is constructed in the following manner : A C (fig. 23), represents a box of about a foot and a half square, shut on every side, except D C; 0 P is a smaller box placed on the top of the greater ; .N is a double convex lens, whose axis makes an angle of 45° with B L, a plane mirror fixed in the box 0 P ; the focal length of the lens is nearly equal to C S S '1', i. e. to the sum of the distances Of the lens from the middle of the mirror, and of the middle of the mirror from the bottom of the larger box. The lens be ing turned toward the prospect would form a picture of it, nearly at its focus ; but the rays being intercepted by the mirror will form the picture as far before the surface as the focus is behind it, i. e. at the bottom of the larger box, a com munication being made between the boxes by the vacant space Q O. The draughtsman then putting his head and hands into the box through the open side, I) C, and drawing a curtain round to prevent the admission of the light, which would disturb the operation, .may trace a distinct outline of the picture that appears on the bottom of the box.
There is another kind of camera ob scura, constructed thus. In the extremi ty of the arm, P Q (fig. 24), that extends from the side of a small square box, B L, is placed a double convex lens, whose axis is inclined in an angle of 45° to a plane mirror B 0 : the focal length of the lens is equal to its distance from the side of the box 0 T ; therefore, when the lens is turned towards the illuminated pros pect, it would project the image on the side 0 T, if the mirror were removed, but this will reflect the image to the side M L, which is as far distant from the mid dle of the mirror, as this is from the side O T ; it is there received on a piece of glass, rough at the upper side and smooth at the lower, and appears in its proper co lours on the upper side of the plate. It is evident that in each of these instruments the image is inverted with respect to the object.
M S is a lid to prevent the admission of light during the delineation of the pic ture, and others for the same purpose are applied to the sides M It and N L.
Dr. Wollaston has recently invented a portable instrument for drawing in per spective, to which he has given the name of Camera Lucida. In this instrument two plane reflectors are fixed at such an gles with regard to each other, that the objects intended to be delineated are seen after reflection from the second mirror, as though they were on the same plane as that whereon the paper is plac ed which is to contain the drawing. These plane reflectors may be either common mirrors with a silver coating at the back of each, or two contiguous faces of a glass prism, in which latter case the image will be produced by what is called prismatic reflection. In either case the most convenient position, in which the reflecting surfaces can be arranged, will be such as will cause the rays proceeding directly from the object and falling as in cident rays upon the first surface, after reflection from thence to the second, to emerge from that second reflecting sur• face in angles of 90 degrees, with the di rection of the original incident rays ; for in these circumstances vertical objects may be projected upon a horizontal plane, and the instrument will be adapt ed to drawing upon a horizontal surface.
Now, if two plane mirrors are used, the incident rays upon the first will make right angles with the emergent rays from the second, when those mirrors are fixed so as to make angles of either 45 or 135 degrees with each other. In this case the mirror which first receives the rays from the object may be entirely silvered at its back; but the second mirror is only to have a sufficient portion silvered to re flect the image of the proposed object to the eye ; and thus to allow the paper, on which the drawing is to be made, to be seen either through an opening of the silvering or past the edges of the same, by one portion of the eye, while the dou ble reflected object is seen in the silver ing by the other portion of the same eye. When prismatic reflection is employed, the prism must not be triangular as usual ly constructed, but quadrangular, and the two reflecting surfaces (to produce an angle of 90 degrees between the first incident and second emergent rays) must make an angle of 135 degrees, while the opposite angle must be one of 90 degrees, and the other two angles may be either respectively equal or unequal at pleasure ; then one of the faces which make right angles with each other is to be turned towards the object or objects to be delineated, and the rays after pass ing through that surface and reflection from the two next faces, will emerge from the fourth under the proposed an gle. The mirrors or other reflecting sur faces are mounted in a proper frame, and supported at a suitable distance from the paper intended to receive the drawing ; and, when necessary, either a double concave or a double convex glass may be fixed in the frame, and property adjust ed to produce distinct vision when the apparatus is used by short-sighted or long-sighted persons respectively. These concave or convex glasses may conve niently be made of twelve inches focal length; the instrument must then be sup ported at the distance of twelve inches from the paper ; a distance which is con venient enough in other respects.
Dr. Wollaston has himself published a description of this instrument, in Nichol son's Philosophical Journal, where he likewise institutes a comparison between the Camera Obscure and the Camera Lu cida. The objections to the Camera Ob scure are, 1. That it is too large to be car ried about with convenience. The Ca mera Lucida is as small and portable as can be wished. 2.1n the former, all ob jects that are not situated 'near the cen tre of view are more or less distorted. In this, there is no distortion ; so that every line, even the most remote from the cen tre of view, is as straight as those through the centre. 3. In that, the field of view does not extend beyond 30°, or at most 35°, with distinctness. But in the Came ra Lucida as much as 70° or 80° might be included in one view.
Dr. Wollaston remarks further, that by a proper use of the same instrument every purpose of the pentagraph may also he answered ; as a painting may be reduced in any proportion required, by placing it at a distance in due propor tion greater than that of the paper from the instrument, In this case a lens becomes requisite for enabling the eye to see at two unequal distances with equal distinctness ; and in order that one lens may suit for all these purposes, there is an advantage in varying the height of the stand according to the pro portion in which the reduction is to be ef fected.