OPTICS. 
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 OT ; 
therefore, when the lens is turned towards 
the illuminated prospect, it would project 
the image on the side O T, if the mirror 
were removed, but tliis will reflect the 
image to the side M L, which is as far dis- 
tant from the middle 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 colours on the upper side of the 
plate. It is evident that in each of these 
instruments the image is inverted with re- 
spect to the object. 
M S is a lid to prevent the admission of 
light during tlie delineation of the picture, 
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 perspec- 
tive, to which he has given the name of 
Camera Lucida. In this instrument two 
plane reflectors are fixed at such angles 
with regard to each other, that the objects 
intended to be delineated are seen after re- 
flection from the second mirror, as thougli 
they were on the same plane as that whereon 
the paper is placed which is to contain the 
drawing. These plane reflectors may be 
eitlier common mirrors with a silver coat- 
ing at the back of each, or two contiguous 
faces of a glass prism, in which latter case 
tlie image will be produced by what is called 
prismatic reflection. In either case tlie 
most convenient position, in which the re- 
flecting surfaces can be arranged, will be 
such as will cause the rays proceeding di- 
rectly from the object and falling as inci- 
dent rays upon the first surface, after re- 
flection from thence to the second, to 
emerge from that second reflecting surface 
in angles of 90 degrees, with the direction 
of the original incident rays ; for in these 
circumstances vertical objects may be pro- 
jected upon a horizontal plane, and the in- 
strument will be adapted 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 emer- 
gent rays from the second, when those mir- 
rors 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 silver- 
ed at its back ; but the second mirror is 
only to have a sufficient portion silvered to 
reflect 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 por- 
tion of the eye, while the double reflected 
object is seen in the silvering by the other 
portion of the same eye. When prismatic 
reflection is employed, the prism must not 
be triangular as usually constructed, but 
quadrangular, and the two reflecting surfaces 
(to produce aft 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 
I’espectively equal or unequal at pleasure ; 
then one of the faces which make right 
angles with each other is to be turned to- 
wards the object or objects to be deli- 
neated, and the rays after passing through 
that surface and reflection from the two 
next faces, wilt emerge from the fourth 
under the proposed angle. The mirrors 
or other reflecting surfaces 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 properly ad- 
justed, to produce distinct vision when the 
apparatus is used by short-sighted or long- 
sighted persons respectively. These con- 
cave or convex glasses may conveniently be 
made of twelve inches focal length ; the in- 
strument must then be supported at the 
distance of twelve inches from the paper ; a 
distance which is convenient enough in other 
respects. 
Dr. AVollaston has himself published a 
description of tliis instrument, in Nichol- 
son’s Philosophical Journal, where he like- 
wise institutes a comparison between the 
Camera Obscura and the Camera Lucida. 
The objection to the Camera Obscura are, 
1. That it is too large to be carried about 
with convenience. The Camera Lucida is as 
small and portable as can be wished. 2 , In 
the former, all objects that are not situated 
near the centre of view are more or less 
distorted. In tliis, there is no distortion ; 
so that every line, even the most remote 
from the centre 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 Camera Lucida as much as 70“ or 80* 
might be included in one view. 
Dr, Wollaston remarks further, that by a 
