OPTICS. 



be seen distinctly. The object is illumi- 

 nated by light "admitted into the tube 

 through a space, P R, adjoining 1 to the 

 speculum ; and the illustration of the ob- 

 ject may be rendered more intense by a 

 concave speculum, A B, which shall re- 

 flect the light so admitted to a focus at 

 the place of the object. 



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 MAGIC LASTERX, 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 DC; O P is a smaller 

 box placed on the top of the greater ; 

 M N is a double convex lens, whose axis 

 makes an angle of 45 with B L, a plane 

 mirror fixed in the box O P ; the focal 

 length of the lens is nearly equal to C S 

 -f- S T, 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, D 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 O : the focal length of the 

 lens is equal to its distance from the side 



of the box T ; therefore, when the lens 

 is turned towards the illuminated pros- 

 pect, it would project the image on the 

 side O 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 R 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 "he 

 reflecting surfaces can be arranged, \vill 

 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- 

 rect! on 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 eacli 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 



