346 Ddscription of the Camera Lmida. 



near the eye, does n.it require to he hirge, I have on many 

 accounts preferred the smallest size that could be executed 

 with corrtctncss, and have had it constructed on such a 

 scale that the leasts are only three-i'ourlhs of an inch in 

 diameter. 



Allhousrh the original design and principal use ot this in- 

 strument avc to facilitate the delineation of objects in true 

 perspective, yet this is by no means the sole purpose to 

 which it is adapted ; for the same arrangement of reflectors 

 may be employed with equal advantage for coijying what 

 has been already drawn, and may thus assist a learner in 

 acquiring; at least a correct outline of any subject. • 



I'or this purpose, the drawing to be eojiicd should be 

 placed, as nearly as may be, at the same distance beiore the 

 instrument that the paper is beneath it ; for in that case the 

 si/.c v>ill be the same, and no lens will be necessary, either 

 to the object or to the pencil. 



By a proper use of the same instrument every purpose of 

 the pentagvapl^ may also be answered, as a painting may be 

 reduced in anv proportion required by placing it at a distance 

 in due proportion greater than iluil of ihe paper from the 

 instrument. In this case a lens becomes requisite for enq- 

 blins: the t-ye to see at two unequal distances with equal di- 

 stinctness ; and, in order that one lens may suit for all these 

 purposes, there is an advantage in varying the height of the 

 stand accorduiLi to the proportion m which the reduction is 

 to be effceted. 



The principles on which the height of the stem is ad- 

 iusted will 1)6 readily understood by those who are accus- 

 tomed to optical considerations. For, as, in taking a per- 

 spective vie>v, the ravs from the paper arc rendered parullel 

 by placing a lens at the distance. of its principal focus from 

 the paper, because the rays from the distant objects are pn- 

 rnlld; so also, when the object seen by reflection is at so 

 short a distance that the ravs leecived from it are in a sensi- 

 ble degree divergent, the rays from the paper shcnild be 

 made to have the &amc degree of divergency, in order that 

 the paper may be seen flistinctly by the same eye ; and for 

 this purpose the lens nm:>t be placed at a distance less than 



il3. 



