60 



UNDULATORY FORCES. LIGHT. 



[lllE CAMERA LUC1DA. 



at once appear on tho ground-glass plate at F ; ami if a 

 piece of paper be placed thereon, an excellent outline or 

 drawing of the image may easily be taken, by simply 

 tracing the lines and shadin 1 ' by mc.uid of a bl&cklcad 

 pencil. 



No difficulty need be experienced by any one in con- 

 structing a camera of this kind, because the position of 

 each part of the apparatus cm easily ho arrived at by 

 trial. Wo may, however, point out two chief matters 

 which should ho attended to ; namely, let the distance of 

 tho lens a front tho mirror be a little beyond that which 

 would permit its focal point to touch the glass ;* and 

 secondly, care must be taken that the glass mirror is 

 properly adjusted, both for receiving the image from the 

 lens, and of transmitting it to the ground-glass plate 

 placed over it. 



The above instrument is an exceedingly valuable one 

 for teaching the art of landscape sketching to youni; 

 persons. The idea of distance and perspective is thus 

 easily acquired ; and as they have not to trust to their 

 imagination, either in sketching or shading, they speedily 

 acquire considerable accuracy in delineating objects, and 

 to an extent far greater than would be at first supposed. 



Our remarks on aberration t will prepare the intelli- 

 gent reader to expect that there will be considerable 

 error in all the external parts of the picture. Tlu's may 

 be avoided by employing a lens of the meniscus form, in 

 place of a convex one. It is, however, more expensive, 

 and less easily obtained than a double convex one. 



A very ingenious application of the camera, on a large 

 scale, is often employed for the purpose of obtaining, on 

 a level surface, extensive views of distant scenery ; and 

 some of these answer the purpose extremely well. One 

 of the best we have yet seen is that of the observatory 

 and museum of Dumfries, in Scotland. By it, views 

 extending over a radius of ten or more miles are accu- 

 rately represented on the receiving-surface of the table. 

 One of these arrangements is represented in Fig. 35. 

 Fig. 3V 



The arrangement is very different to the simple camera 

 wo have just described. In Fig. 35, a represents a re- 

 flector, on which the distant view is first received ; the 

 light is thence transmitted through the lens 6, and at last 

 received on a table covered evenly with white paper, as 

 at c. The spectators should be placed, with the table, in 

 a room or tent from which all light is excluded but that 

 which passes through the lens. An arrangement of this 

 sort can be easily fitted in the roof of an attic ; and if the 

 reflector be made so as that it can be turned round in any 

 direction, or the angle of its elevation altered, the spec- 

 tators may have either a view of distant or proximate 

 objects in any direction around them. 



See ante, f. 47. + Ante, P- 44. 



This form of the camera is highly applicable to the 

 purposes of tho artist ; and if the lenses and arrangen. 

 are carefully fitted, tho picture produced on the i 

 screen is of great beauty :m 1 a '-curacy. As an illn 

 tion of this, wo may observe, that by means of the camera 

 to which wo have already alluded, we were enabled to 

 tho ridges of hills and the windings of a very dis- 

 tant river, in a fur more distinct manner than the painter 

 or sketchor over attempts to delineate them either on 

 canvas or paper. 



The camera hu-ida, which consists of a four-sided glass 

 prism, is a very iii'_nioiis and simple contrivance, v. 

 may also bo used by artists for sketching purposes. One 

 of those is represented in the succeeding cut. 



In Fig. Pi g . 38. 



a a, a a', repre- 

 sent the direc- 

 tion of the rays 

 proceeding from 

 an object, ami 

 thoirsubsequent 

 reflection both to 

 the eye and, ap- 

 parently, to a 

 piece of paper 

 placed beneath 

 the instrument ; 

 6 c and b d are 

 two sides of the 

 glass prism ; the 

 angle c b d being 

 a right angle. 

 The other two 

 faces of the glass are cut at different angles ; e being 

 at one of 135, and / at an angle of 67 . The rays 

 a a first fall on the face /, and being thence reflected 

 to e, reach the eye, which seems to observe their image 

 on the sheet of paper, </, placed beneath the prism. 

 The spectator, looking through the glass, is thus enabled 

 to sketch an imago on a surface placed parallel to the 

 rays of light proceeding from the object. On careful 

 inspection, the reader will perceive some similarity to the 

 camera obscura which we previously described the eye, 

 in fact, receiving the rays as did the ground-glass plate. 



We sluill hero include a description of a well-known 

 optical toy, called the Multiplying Glass. It lias one 

 flat surface, on the other side of which numerous facets 

 or planes are cut. It may be described as a plano-convex 

 lens, whose curved surface is formed of a variety of 

 planes ; and, like that instrument, the multiplying glass 

 produces its effects by the refraction of the rays of light 

 incident on one of its surfaces. Fig. 37 represents one 

 of these arrangements with 

 three planes. repiv 

 the flat or entire plane sur- 

 face of the glass, and bed 

 three planes cut on the op- 

 posite surface. If a ray of 

 light pass from an object 

 e to the eye of the specta- 

 tor, tlirough a c, it will not 

 undergo refraction, for reasons already stated ;J but the 

 rays e b and e d, falling obliquely on tho planes 6 and d, 

 will suffer refraction, and will be therefore viewed at a 

 ditfcrent angle by the eye than that which passes perpen- 

 dicularly through, as at a c e. Of course, the same rea- 

 soning applies to any number of facets cut on the glass ; 

 and as the number of these will increase the number of 

 angles under which one object will appear to tho eye of 

 tho observer, so the appearance of the object will be 

 proportionably multiplied. 



THE MAGIC LANTERN, PHANTASMAGORIA, 

 DISSOLVING VIEWS. CHROMOTROPES, AND 

 HLYSIOSCOPE. 



THERK are few optical instruments which are of so 



amusing a character as those named at the head of this 



I 8w ante, p. 49. 



