of the Light emitted by Luminous Bodies. 13 



strong, thick piece of well-hammered plate brass, which 

 plate of brass may be afterwards fastened to the 

 bottom of the box by means of one strong screw. In 

 this manner two of my best instruments are con- 

 structed. And, in order to secure the cylinders still 

 more firmly in their vertical positions, they are fur- 

 nished with broad flat rings or projections, where they 

 rest upon the brass plate ; which rings are iV of an 

 inch thick, and equal in diameter to the projection of 

 the wing of the cylinder, to the bottom of which they 

 afford a firm support. (See Plate I., Fig. i.) These 

 cylinders are likewise forcibly pushed, or rather pulled, 

 against the brass plate upon which they rest, by means 

 of compressed spiral springs placed between the under 

 side of that plate and the lower ends of the cylinders. 



Of whatever material the cylinders be constructed, 

 and whatever be their forms or dimensions, it is abso- 

 lutely necessary that they, as well as every other part 

 of the photometer except the field, should be well 

 painted of a deep black, dead colour. That, and that 

 alone, will prevent the inconveniencies which would 

 otherwise arise from reflected light and from the pres- 

 ence of too great a number of visible objects. 



In order to move the lights to and from the pho- 

 tometer with greater ease and precision, I provided two 

 long and narrow but very strong and steady tables, in 

 the middle of each of which there is a straight groove, 

 in which a sliding carriage, upon which the light is 

 placed, is drawn along by means of a cord which is 

 fastened to it before and behind, and which passing 

 over pulleys at each end of the table goes round a cyl- 

 inder, which cylinder is furnished with a winch, and 

 is so placed, near the end of the table adjoining the 



