NATURAL PHILOSOPHY. 155 



to reduce the limits of probable error. The device for marking time is an 

 application of the revolving glass prism. Thus, immediately in front of the 

 object-glass of the camera, a glass prism, of small angle and sufficient area to 

 cover the entire aperture, is made to rotate at an accurately measured rate 

 of say twenty -five revolutions per second. The prism may be replaced by an 

 eccentric lens, or the object-glass itself may revolve on a slightly eccentric 

 axis. The consequence will be that the image of a fixed star in any part of 

 the field of view will traverse the circumference of a circle every twenty-fifth of 

 a second, and the image of a shooting star will combine this motion with its 

 motion of translation. If the photographic surface retain a visible impres- 

 sion of the looped curve or the waved curve which will thus be produced, 

 then, neglecting for the present the small effects of optical distortion, the 

 line drawn midway between the two straight or regularly curved lines 

 between which the looped or waved curve oscillates, will represent the 

 apparent track of the meteor, and the points where it intersects the looped 

 or waved curve, if they be translated along this middle line through a space 

 equal to the optical displacement of the meteoric image, will show the 

 apparent place occupied by the meteor at points of time separated by the 

 equal intervals of one-fiftieth of a second. 



In the above statement I have supposed only a single camera, but it will 

 probably be impossible in this way to command a sufficient extent of the 

 heavens. A system of many cameras may, however, be formed, so arranged 

 that their several optic axes shall cross in a common point in front of the 

 object-glasses. The object-glasses may thus be approximated as closely as 

 we can desire, and the several revolving prisms, or eccentric lenses, may 

 have a common geared connection, and the backs of the cameras will be 

 readily accessible for the renewal of plates. 



The observer, after having made the necessary adjustments, will be 

 charged with the sole duty of watching for meteors in the region covered by 

 his system of cameras, and at the appearance of a meteor will touch a 

 spring, so contrived as to cause the instant unveiling of all the cameras of 

 the system, and on the extinction of the meteor will promptly replace the 

 screen. The expense and trouble of the process will be certainly great, but 

 will not be disproportionate to the importance of the object in view. Only 

 let us have a photographic surface that will give a visible trace of the 

 meteor's path, in the face of exposure to the light of the sky during the time 

 of the meteor's visible flight, and then success, as regards the*tattainment of 

 an accurate record, will be nearly certain, and we should not hesitate at the 

 expense and trouble. 



If, upon suitable trials made upon the fixed stars, and upon shooting stars 

 themselves, we find ourselves in possession of sufficient photographic power, 

 there is no reason why an organized system of observations should not be 

 instituted. A moderate degree of accuracy in the absolute determination of 

 the orbits, except when they make a near approach to the parabola, will be 

 sufficient to answer all the questions of interest that will be likely to arise, 

 upon which a knowledge of the orbits would have any bearing. Whether 

 the November meteors, for instance, move through regions that would 

 identify them with the Zodiacal light, according to the theory of the late 

 Prof. Olmsted, is a question that would receive an absolute determination. 



Charcoal Photographs and Photographic Enamels. Indestructible charcoal 

 photographs are now produced by exposing gelatine and bi-chromate of 

 potash to the action of light, and then exposing the surface to steam. The 



