408 



THE POPULAR EDUCATOR. 



RECREATIVE SCIENCE. IX. 



AJffTTSING OPTICAL INSTRUMENTS ILLUSTRATING THE LAWS 

 OP REFLECTION AND REFRACTION THE CAMERA OB- 



SCUEA (continued). 



THE rise and progress of any special art demanding the aid of 

 science is always an instructive theme for the youthful student 

 the beginning and the end seem to be so utterly different and 

 widely separated. When the artist constructed with elaborate 

 care his travelling camera obscura, and sketched upon the little 

 table the picture painted by the aid of his optical apparatus, he 

 little thought that the time would come when the same light 

 that caused the picture to 

 appear upon the ground 

 glass or sheet of paper 

 would leave its own silent 

 impress on the medium 

 carefully prepared by che- 

 mical means, and that light 

 might save the would-be 

 limner the trouble and time 

 reqniredf or art-studies, and 

 do for him more than the 

 most delicate hand, skilled 

 in elaborate tracings, could 

 possibly perform. One of 

 the foundations of thcbeau- 

 tiful art of photography 

 was the " camera obscura." 

 The desire to fix the 

 pretty, and at that time 

 evanescent, picture must 

 from an early period have 

 engaged the attention of 

 thoughtful minds. And 

 here the construction of a 

 complete travelling camera 

 obscura and dark box, in- 

 vented by Gravesande and 

 described by Jean fitienno 

 Montucla, is interesting, 

 because we have its parallel 

 in the dark tent and more 

 portable travelling equi- 

 page of the photographer. 

 In his essay on " Perspec- 

 tive," Gravesande thus de- 

 scribes his camera obscura: 

 "This machine is shaped 

 almost like a ' sedan chair ; ' 

 the top of it is rounded off 

 towards the back part, and 

 before it swells out into 

 an arch at about the mid- 

 dle of its height." 



1. The board A in the 

 inside (Fig. 1) serves as a 

 table ; it turns on two iron 

 hinges fastened to the fore 

 part of the machine, and 



is supported by two small chains, so that it may be raised to : 

 allow the operator to enter the machine by the door R. 



2. On the outside of the back of the machine are fixed four j 

 small staples, c, c, c, c, in which slide two pieces of wood, D E, j 

 b E, three inches in breadth, and through these pass two other j 

 pieces, serving to keep fast a small board F, which by their j 

 means can be moved backwards and forwards. 



3. At the top of the machine is a slit o Q, nine or ten inches ! 

 in length, and four in breadth, to the edges of which are affixed j 

 two rules in the form of a dovetail ; between these slides a ; 

 board of the same length having a round hole, of about three I 

 inches in diameter, in the middle, furnished with a nut, that ! 

 serves to raise and lower a tube, G, about four inches in height, < 

 which has a screw corresponding to the nut. This tube is , 

 intended for a convex glass lens. 



4. The movable board, above described, supports a square i 

 box x, about seven and a-half inches in breadth and ten in ; 

 hsight, the fore part of which can be opened by a small door, B- 



and in the back part of the box, towards the bottom, is a square 

 aperture N, of about four inches in breadth, which may be shut 

 at pleasure by a sliding board, i. 



5. Facing this square aperture, N, is a slit which occupies 

 the whole breadth of the box. It serves for fixing on the box 

 a plain mirror, H, which slides between two rules, so that the 

 angle it makes with the horizon towards the door is 112 

 degrees, or four-fifths of a right angle. 



6. The same mirror, when necessary, may be placed in a 

 direction perpendicular to the horizon, as represented at H, 

 by means of a small iron plate adapted to one of its sides, 

 and which may be screwed fast by means of a nut. 



7. Within the box x is 

 another small mirror, L, 

 fixed a little above the slit 

 o G, and which, being 

 drawn up or pushed down 

 by the small rod s, may be 

 inclined to the horizon at 

 any angle whatever. 



8. That the machine may 

 be supplied with air, a tube 

 of tin plate or other metal, 

 bent at both ends, maj be 

 fitted into one of the sidss, 

 and this will give access to 

 the air without admitting 

 light. But if this should 

 not be sufficient, a small 

 pair of bellows to be 

 moved by the foot may be 

 placed below the scat, and 

 in this mariner the air may 

 be continually renewed. 



The different uses to 

 which this camera may 

 be applied are thus de- 

 scribed : 



(1.) To represent objects 

 ifi, their -natural situation. 

 When objects are to be 

 represented in this machine, 

 extend a sheet of papei 

 on the table, or, rather, 

 stretched on a frame ; or 

 you may employ a piece of 

 strong card, and fix it in 

 such a manner as to be- 

 come immovable. In the 

 tube G (Fig. 1) place a 

 convex lens, the focus of 

 which is nearly equal to 

 the height of the machine 

 above the table ; open the 

 back of the box x, and, 

 having removed the mirror 

 H, as well as the board F, 

 and the rods D E, D E, in- 

 cline the movable mirror 

 L till it makes with the 



horizon an angle of nearly 45, if you intend to represent 

 objects at a considerable distance, and which form a perpen- 

 dicular landscape. When this is done, all those objects which 

 transmit rays to the mirror L, so as to be reflected on the 

 convex glass, will appear painted on the paper on the table A; 

 the point where the images are most distinct may be found, 

 if the tube which contains the lens be lowered or raised, by 

 screwing it up or down. 



By these means any landscape, view of a city, etc., may be 

 exhibited with the greatest precision. 



(2.) To represent objects in such a, manner as to make that 

 which is on the right appear on the left, or vice versa. The 

 box x being in the situation represented in Fig. 1, open the 

 door B, and having placed the mirror H in the slit, and in the 

 situation already mentioned at No. 5, raise the mirror L till it 

 makes with the horizon an angle of 22^ degrees ; if the fore part 

 of the machine be then turned towards the objects to be 

 represented, which we have supposed to be at a considerable 



1. 



