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THE POPULAR EDUCATOR. 



RECREATIVE SCIENCE. VIII. 



AMUSING OPTICAL INSTRUMENTS ILLUSTRATING THE ! 

 LAWS OF REFLECTION AND REFRACTION THE 

 CAMERA-OBSCURA. 



IT would, of course, be difficult to say who was the first to 

 conceive the idea of collecting and publishing a description, with 

 appropriate engravings, of known experiments in physical science. 

 That the idea is not new, and was thought useful in bygone 

 times, is shown by the absurd though somewhat amusing " Re- 

 creation in Mathematics and Natural Philosophy," written nearly 

 two hundred years ago, by Jacques Ozanam, and subsequently 

 revised, enlarged, and improved by Jean fitienne Montucla, in 

 the year 1793, in 

 his new edition of 

 the " Methodical 

 Encyclopaedia of 

 Amusements in 

 Mathematical 

 and Physical 

 Science," and de- 

 dicated to the 

 Most Serene Re- 

 public of Venice. 

 In the preface the 

 reader is told 

 " that the useful 

 is combined 

 nearly always 

 with the agree- 

 able, and that he 

 may instruct and 

 amuse himself;" 

 no exaggerated 

 statement, be- 

 cause the famous 

 Dr. Charles Hut- 

 ton, the mathe- 

 matician, pub- 

 lished a transla- 

 tion of the above 

 work in 1803, 

 and subsequently 

 another edition in 

 1814. In the pre- 

 sent paper some 

 careful reproduc- 

 tions of Mon- 

 tucla's original 

 diagrams will be 

 given. 



Nearly the first 

 optical amuse- 

 ment proposed is 

 how " to exhibit, 

 in a darkened 

 room, external 

 objects, in their 

 natural colours 

 and proportions;" 

 and in this de- 

 scription the principles and construction of the camera-obscura, 

 or darkened chamber, are fully developed : " Close the door 

 and darken the windows of the chamber, so that no light can 

 enter, except that which can pass through a very small and well- 

 cut hole in the shutter or other wood- work used to darken the 

 window facing some public street or pretty country landscape. 

 Hang against the wall opposite the hole in the shutter a very 

 white cloth, and if the external objects are well illuminated, 

 and the chamber kept very dark, they will paint themselves on 

 the wall or cloth with their natural colours, in a reversed posi- j 

 tion." 



The experiment conducted in this very simple manner sue- j 

 ceeds sufficiently well to surprise those who may witness it 

 for the first time ; but it is made much more striking with the i 

 help of a glass lens. Adapt to the hole of the shutter which i 

 in this case must be some inches in diameter a tube, having j 

 at its internal extremity a convex lens of four, five, or six feet ' 



focus. By holding a sheet of white paper or the cloth at that 

 distance from the lens in a line perpendicular to the axis of the 

 tube, the outer objects, such as the trees, houses, or persons 

 promenading, are painted on it with wonderful fidelity, distinct- 

 ness, and brilliancy of colouring, so much so that even the 

 features of persons may be clearly distinguished. 



A little of the effect is certainly destroyed by the inversion 

 of the painting, and although there are various ways in which 

 this may be corrected, it cannot be done without reducing the 

 field of the picture and injuring the sharpness of the outlines 

 of the figures projected on the paper or cloth. Nevertheless, if 

 the spectator wishes to have the convenience of seeing the 

 objects in their right position, he may proceed as follows : 



Place a plane 

 mirror, inclined 

 at an angle of 45, 

 at nearly half the 

 focal distance of 

 the lens, in such 

 a way that it may 

 reflect down- 

 wards the rays 

 proceeding from 

 the lens ; arrange 

 horizontally below 

 it a sheet of paper, 

 upon which the 

 image of the ex- 

 ternal object (in 

 this case a bust) 

 will appear. The 

 image will be seen 

 in its natural po- 

 sition and upright 

 by all those who 

 look at it in a 

 certain position 

 viz., with their 

 backs turned to- 

 wards the opening: 

 in the shutter con- 

 taining the lens. 

 The inverted 

 image is shown all 

 Fig. 1, and the 

 cause of the re- 

 versal of the posi- 

 tion of the bust is 

 evident, from the 

 fact that the rays 

 cross at the aper- 

 ture in the shutter. 

 When the sheet, 

 of paper is laid; 

 out on a table, 

 arranged to rise 

 and fall by rack- 

 work, or any other 

 simple means, 

 there will be 

 painted upon it 



an exact picture of all the objects before the window. People,, 

 cows, horses, sheep, etc., are seen with the same movements and 

 gestures which they exhibit under ordinary circumstances ; 

 whilst the trees, the sky, and clouds are painted in their natural 

 colours, either calm or agitated by the wind as the case may 

 be. The use of the rack-work is to enable the spectator to focus 

 the more distant objects, and if great sharpness in the image is 

 desired, the top of the table receiving the picture should be 

 made of plaster of Paris, and hollowed out to the same curve 

 as the lens. 



A PORTABLE CAMERA OBSCURA. 



Baptista Porta's amusing optical instrument admits of many 

 modifications, and Fig. 2 represents another, and at one time 

 very popular, form of the camera. 



They were formerly much used by artists, on account of their 

 very convenient dimensions, and were constructed of mahogany, 

 and of various sizes, some so small as to be carried in the pocket. 



