Elements of natural philosophy (Volume 2-3) . dent. This little instrument is the morevaluable, because it gives, by an easy process, the magni-fying power of any telescope, however complicated. CAMERA LUCIDA. Fig. 61. Used to copyfrom nature : §87. This little instrument, the invention of Dr. , is of great assistance in drawing from its simplest form, it consists of a glass prism, a sectionof which is represent-ed by ABCD, withone right angle at A^and the opposite angle(7,135°. Kays proceed-ing from a point of anyobject /#, in front of theface A D, enter thisf
Elements of natural philosophy (Volume 2-3) . dent. This little instrument is the morevaluable, because it gives, by an easy process, the magni-fying power of any telescope, however complicated. CAMERA LUCIDA. Fig. 61. Used to copyfrom nature : §87. This little instrument, the invention of Dr. , is of great assistance in drawing from its simplest form, it consists of a glass prism, a sectionof which is represent-ed by ABCD, withone right angle at A^and the opposite angle(7,135°. Kays proceed-ing from a point of anyobject /#, in front of theface A D, enter thisface without undergo-ing any material devi-ation, and being re-ceived in successionbv the faces D C andCB within the limits oftotal reflexion, they are reflected, and finally leave the face B A, in nearly the samestate of divergence as when they left the object S. Theeye E, being so placed that the edge B of the prism Explanation;shall bisect the pupil, will receive these rays and bringthem to a focus r, on the retina, at the same time that. Essential parts 262 NATURAL PHILOSOPHY. Action of thecamera lucida informing images; Fig. 61. B & —.•*£- Linear dimensions ofobject and image. w Use of convex orconcave lenswith the cameralucida; The instrumenthas variousforms. it will receive through the half of the pupil not coveredby the prism, rays proceeding from the point P, of apencil, placed below on a sheet of paper, and bring themalso to the same focus r; so that the point in the ob-ject and point of the pencil will appear to coincide onthe paper, the wholeof which will be seen B through the uncoveredhalf of the pupil, anda picture of the objectmay thus be traced bybringing the pencilin succession in ap-parent contact withits various parts. The linear dimen-sions of the picturewill be to those of theobject, as the distance of the camera from the paper,to its distance from the object, nearly. If the paper be very near, the eye may not havepower to bring the rays proceeding fr
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