298 



CAMERA. 



Camera this portion of the eye, while the paper and pencil 

 lucida. are 8een pagt t j ie e( jg e O f t he prism by the remainder 



of the pupil. 



In order to avoid the inconvenience that might arise 

 from an unintentional motion of the eye, the relative 

 quantities of light to be received from the object, 

 and from the paper, are regulated by a small hole in 

 a piece of brass, which by moving on a centre at c, 

 Fig. 6. is capable of adjustment to every inequality 

 of light that is likely to occur. 



Since the size of the whole instrument, from being 

 so near the eye, does not require to be large, I have, 

 on many accounts, preferred the smallest size that 

 could be executed with correctness, and have had it 

 constructed on such a scale, that the lenses are only 

 ^ths of an inch in diameter. 



Though the original design and principal use of 

 this instrument is to facilitate the delineation of ob- 

 jects 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 copying what has been already 

 drawn, and may thus assist a learner in acquiring at 

 least a correct outline of any subject. 



For this purpose, the drawing to be copied should 

 be placed as nearly as may be at the same distance 

 before the instrument that the paper is beneath the 

 eye-hole ; for in that case the size will 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 pentagraph may also be answered, as 

 a painting may be reduced in any proportion requir- 

 ed, by placing it at a distance in due proportion 

 greater than that of the paper from the instrument. 

 In this case a lens becomes requisite for enabling the 

 eye to see at two unequal distances with equal dis- 

 tinctness ; and in order that one lens may suit for all 

 these purposes, there is an advantage in carrying the 

 height of the stand according to the proportion in 

 which the reduction is to be effected. 



The principles on which the height of the stem is 

 adjusted will be readily understood by those who are 

 accustomed to optical considerations. For as in tak- 

 ing a perspective view, the rays from the paper are 

 rendered parallel, by placing a lens at the distance of 

 its principal focus from the paper, because the rays 

 received from the distant objects are parallel ; so also 

 when the object seen by reflection is at so short a 

 distance that the rays received from it are, in a ct r- 

 tain degree, divergent, the rays from the paper should 

 be made to have the same degree of divergency, in 

 order that the paper may be seen distinctly by the 

 same eye ; and for this purpose, the lens must be 

 placed at a distance less than its principal focus. The 

 stem of the instrument is accordingly marked at cer- 

 tain distances, to which the conjugate foci are in the 

 several proportions of 2, 3, 4, &c. to 1, so that dis- 

 tinct vision may be obtained in all cases, by placing 

 the painting proportionally more distant. 



By transposing the convex lens to the front of the 

 instrument, and reversing the proportional distances, 

 the artist might also enlarge his smaller sketches with 

 every desirable degree of correctness, and the natu- 



lucida. 



CIX. 

 fig. 7, 



ralist might delineate minute objects in any degree 

 magnified. 



Since the primary intention of this instrument is 

 already, in some measure, answered by the camera 

 obscura, a comparison will naturally be made be- 

 tween them. 



The objections to the camera obscura are, 

 1st, That it is too large to be carried about with 

 convenience. 



The camera lucida is as small and portable as can 

 be wished. 



2dly, In the former, all objects that are not si- 

 tuated near the centre of view, are more or less dis- 

 torted. 



In. this, there is no distortion ; so that every line, 

 even the most remote from the centre of view, is as 

 strait as those through the centre. 



3dly, 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." 



It is obvious, that the preceding contrivance may New tele- 

 be applied to a telescope, for the purpose of ta- scopical 

 king sketches of the different objects that may be camera lu 

 contained within the field of view ; but as it is only 

 a small portion of a landscape, or of any large ob- 

 ject, that can be seen at once through a telescope, 

 it would be desirable to have some contrivance by 

 which the objects seen in different fields of view, and 

 sketched upon the same piece of paper, might be all 

 connected with each other into one landscape. This, 

 however, can be done only to a certain extent, as 

 will appear from Plate CIX. Fig. 7. Let AB be 

 the direction of the telescope, which, when placed 

 upon a suitable stand, can be moved round the axis 

 O in a horizontal plane, B b b' ; B, the extremity of 

 the eye-tube at which the prism of the camera lu- 

 cida is fixed ; MN, the paper, lying in a horizontal 

 position ; and a b, a' b', successive positions of the 

 telescope in a plane parallel to MN. Let EF be the 

 field of view of the telescope, when seen on the pa- 

 per by reflection from the prism ; then the instru- 

 ment must be so constructed, that when the telescope 

 is in the position a b, and directed to the part of the 

 landscape immediately adjacent to that which is con- 

 tained in the field EF, the field of view FG, when seen 

 by reflection from the prism, must be in contact with 

 EF. When this happens, we have B b z= C c, and 

 the angle BF b = EBF the angle subtended by the 

 field of view ; but it is obvious, that when the tele- 

 scope is moved from the position A B into the position 

 a b, its angular motion round O, viz. the angle BO b f 

 is equal to the angle comprehended by the field of 

 view, that is, to the angle BFE ; therefore, in the 

 triangles OB b, BF 6, we have the angles at O and 

 F equal, and the side B b common ; and consequent- 

 ly the side OB is equal to the side BC. From this 

 it follows, that, in order to have the successive fields 

 of view EF, FG, GH, all joined to each other, or 

 at their proper relative distances, the distance of the 

 eye from the paper must be equal to its distance from 

 the centre of motion O round which the telescope 

 revolves. The telescope should therefore be placed 

 pou a stand so constructed, that the centre of mo- 



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