of the Light emitted by Luminous Bodies. 45 



three holes through it, in which three cylindrical pillars 

 pass, which stand on the bottom of the carriage, and 

 are firmly fixed in it. The platform is attached to 

 these three pillars at any height above the bottom of 

 the carriage, by means of small horizontal screws, 

 which can be made to press against the pillars. These 

 screws are fixed in large hollow knobs of wood which 

 are fixed to the platform, just over the holes, in such a 

 manner that each pillar passes through the axis of one 

 of these knobs. 



One of these knobs, together with the end of the 

 screw by which it is fastened to the pillar, is repre- 

 sented in the bird's-eye view of the carriage b, and 

 another in that c, Fig. 3. The reason w^hy the other 

 two knobs belonging to each of these carriages are 

 not seen is this : they are hid by a flat narrow piece 

 of wood (represented in the figure) which, passing 

 from the top of one of the two front pillars of the 

 carriage to the other, serves to make those pillars more 

 steady. A front view of the three knobs belonging to 

 each of the carriages may be seen in the next figure. 



d and e are the winches by means of which the slid- 

 ing carriages, b and c, are occasionally brought nearer 

 to and carried farther from the field of the photom- 

 eter. The strong wires stretched along upon each 

 side of each of the long tables on which the carriages 

 slide are represented in this figure, as also the cord 

 stretched along the middle of each table, and passing 

 over pulleys at each end of it, and round the cylinder 

 of the winch, which serves for drawing the carriage 

 backwards and forwards. 



The two ends of this cord are united under the table, 

 forming of the whole a kind of band, which is kept at 



