Camera 

 lucid*. 



CAM 



ire sometimes only penetrated by the colour,through 

 ) ilf their thickness; so that when pieces of it are ren- 

 dered opaque by the preceding process, the trans- 

 parent part of it that was not penetrated by the colour 

 becomes white, and two differently coloured layers 

 are thus obtained, xvithout the process of soldering. 

 The white part of the glass is then ground down to 

 the requisite thickness, and the figures are cut upon 

 it either in alto or basso relievo by the common pro- 

 cess. 



If the subject to be engraved is only a simple head, 

 it may be first cut out, and then fixed upon 9 piece of 

 glass of a different colour. The head is first attached 

 with gum to the piece of glass, and after the ad- 

 hesion is sufficiently firm, the surface of the glass 

 containing the head is laid upon a bed of tripoli, and 

 pressed upon it as if a good impression of the head 

 were required. The glass and the head are allowed 

 to rest upon the tripofi till it is quite dry, and as soon 

 as this happens, the whole is put into a furnace and 

 exposed to an intense heat. The gum is then burnt 

 off, and the head is soldered to the piece of coloured 

 glass upon which it was placed. The hair of the 

 figure may be afterwards coloured, by using a little 

 of the solution of silver in the spirit of nitre, and again 

 exposing the head to a proper heat For further in- 

 formation on this subject, see Essai* sur L? Art des 

 ancicns, de jotndre par le fusion deux especes de 

 verre, pour le gravure en relief par M. Gerhardt, 

 Conseiller Prive des Finances au departement des 

 mines du Roi de Prusse. (?r) 



CAMERA LUCIDA, the name of an instrument 

 for enabling those who have no knowledge of the 

 art of drawing, to take sketches either of natu- 

 ral or artificial objects. This ingenious and useful 

 contrivance was invented in the year 1807, by W. H. 

 Wollaston, M. D. Sec. R. S., a philosopher to whom 

 science is indebted for numerous inventions and dis- 

 coveries. As this instrument is of extensive use, its 

 ingenious inventor has very properly secured the ex- 

 clusive sale of it by patent. The description which 

 Dr Wollaston has given of the Camera lucida is 

 so short and perspicuous, that we shall make no apo- 

 logy for giving it in his own words. We shall then 

 conclude the article with an account of an instru- 

 ment founded on the same principle. 



" While I look directly down at a sheet of paper 

 on my table, if 1 hold between my eye and the pa- 

 per a piece of plain glass, inclined from me down- 

 wards at an angle of 45, I see by reflection the 

 view that is before me, in the same direction that I 

 see my paper through the glass. I might then take 

 a sketch of it ; but the position of the object would 

 be reversed. 



To obtain a direct view, it is necessary to have 

 two reflections. The transparent glass must for this 

 purpose be inclined to the perpendicular line of sight 

 only the half of 45, that it may reflect the view a 

 second time from a piece of looking glass placed be- 

 neath it, and inclined upwards at an equal angle. 

 The objects now appear as if seen through the pa- 

 per in the same place as before ; but they are direct 

 instead cf being inverted, and they may be discerned 

 in this manner sufficiently well for determining the 

 principal positions. 



VOL. V. PART I. 



297 



CAM 



PlAT 



CIX. 

 Fig. 4. 



The pencil} however, and any object which it it Camera 

 to trace, cannot both be seen distinctly in the same 

 state of the eye, on account of the difference of their 

 distances, and the efforts of successive adaptation of 

 the eye to one or to the other, would become pain- 

 ful if frequently repeated. In order to remedy this 

 inconvenience, the paper and pencil may be viewed 

 through a convex lens of such a focus, as to require 

 no more effort than is necessary for seeing the distant 

 objects distinctly. These wifl then appear to cor- 

 respond with the paper in distance as well as direc- 

 tion, and may be drawn with facility, and with any 

 desired degree of precision. 



This arrangement of glasses will probably be best * 

 understood from inspection of Plate CIX. Fig. 4. 

 a b is the transparent glass ; b c the lower reflec- 

 tor ; bd& convex lens (of twelve inches focus) ; e 

 the position of the eye ; and fg h e the course of 

 the rays. 



In some cases a different construction will be pre- 

 ferable. Those eyes, which wfthout assistance are 

 adapted to seeing near objects alone, will not admit 

 the use of a convex glass ; but will, on the contrary, 

 require one that is concave to be placed in front, to 

 render the distant objects distinct. The frame for a 

 glass of this construction is represented at tic, Fig. 6. Fifl> * 

 turning upon the same hinge at // with a convex glass 

 in the frame Im, and moving in such a manner, that 

 either of the glasses may be turned alone into its 

 place, as may be necessary to suit an eye that is long 

 or short sighted. Those persons, however, whose 

 sight is nearly perfect, may at pleasure use either of 

 the glasses. 



The instrument represented in that figure differs 

 moreover in other respects from the foregoing, which 

 I have chosen to describe first, because the action 

 of the reflectors there employed would be more ge- 

 nerally understood. But those who are conversant 

 with the science of optics, will perceive the advan- 

 tage that may be derived in this instance from pris- 

 matic reflection ; for when a rav of light has entered 

 a solid piece of glass, and falls from within upon 

 any surface, at an inclination of only twenty-two 

 or twenty-three degrees, as above supposed, the re- 

 fractive power of the glass is such as to suffer none 

 of that light to pass out, and the surface becomes 

 in this case the most brilliant reflector that can be 

 employed. 



Fig 5. represents the section of a solid prismatic Fig. 5. 

 piece of glass, within which both the reflections re- 

 quisite are effected at the surfaces a b, b c, in such a 

 manner that the ray^/g, after being reflected first at 

 g, and again at //, arrives at the eye in a direction h e 

 at right angles tofg. 



There is another circumstance in this construction 

 necessary to be attended to, and which remains to be 

 explained. Where the reflection was produced by a 

 piece of plain glass, it is obvious that any objects 

 behind the glass (if sufficiently illuminated) might be 

 seen through the glass as well as the reflected image. 

 But when the prismatic reflector is employed, since 

 no light can be transmitted directly through it, the 

 eye must be so placed that only a part of its pupil 

 may be intercepted by the edge of the prism, as at 

 o, Fig. 5. The distant objects will then be seen by 



