128 , PHYSICS. 
f. Of Optical Instruments. 
Optical instruments are divided into catoptric, or those in which mirrors 
are used; dioptric, or those employing Jenses; and cata-dioptric, those in 
which mirrors and lenses are combined. <A single exception to this 
classification is perhaps to be found in the camera lucida of Dr. Wollaston. 
This is an instrument much used in obtaining the outlines of an object. It 
consists of a four-sided prism, abed (pl. 21, fig. 56), having a right 
angle at b, an obtuse angle of 135° at d, and acute angles of 674° at a and c. 
The prism is to be turned with the side bc towards the object. A ray coming 
from the object in the direction of z, enters the side be perpendicularly, 
passing then without refraction to de. Here it experiences total reflection 
from dc, in the direction rr, and again from ad in the direction rp, 
perpendicular to its original direction, zr. The image of the object will 
then be reflected to the eye placed at p, and as we see objects in the 
direction in which rays from them enter the eye, the object in question will 
appear in a direction pr. The eye must be so placed that the pupil may 
be bisected by the edge, a, of the prism; the image then being seen by the 
anterior half of the retina, and the point of a pencil by the posterior, the 
outlines of the former may readily be traced by the pencil. The prism 
is generally inclosed in a box, and erected on a frame (/jfig.-104) with 
various subsidiary apparatus. 
The object of the camera lucida is also attained by the camera obscura, 
an instrument. which may have a great many different constructions. The 
simplest of these is shown in fig. 58. This consists of a tolerably high 
pyramidical box, truncated above. Through the top passes a tube containing 
a convex lens. Over the upper end of the tube is placed a plane mirror, 
forming an angle of 45° with it. Rays coming from external objects are 
reflected by the mirror through the tube, falling finally upon a sheet of paper 
placed on the bottom of the box. A diminished image is thus produced by 
the help of the lens, and access being had by means of a hole in the side, the 
outlines may readily be traced with a pencil. 
Another form of the camera obscura is exhibited in fig. 57. This consists 
of a box, ABCD, with a narrow neck, abcd, in which a convex lens, bc, is 
attached. This is turned towards the object in question, rays from which, 
after passing through the lens, are reflected by the mirror, ef, to a plate of 
eround glass at ik. A distinct image of the object will here be visible. 
The glass gh serves to keep off extraneous light. The camera clara differs 
from this arrangement in having a large Jens in place of the ground 
glass. Upon this the image is depicted in sharp outlines and lively 
coloring. 
Among the more important optical instruments are, the microscope for 
viewing very minute objects at short distances, and the telescope for viewing 
large objects at great distances. A simple microscope is one in which the 
first image formed is received unchanged on the retina. It may consist of 
several lenses; generally, however, of but one. The common lens isa single 
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