362 PHILOSOPHICAL TRANSACTIONS. [aNNO l/Q'l. 



alterations in the instruments. And, in the first place, the shadows, instead of 

 being thrown on a paper spread out on the wainscot, or side of the room, are 

 now projected on the inside of the back part of a wooden box, 7^ inches wide, 

 lOi inches long, and 3i inches deep, in the clear, open in front to receive the 

 light, and painted black on the inside, in every part except the back, on which 

 the white paper is fastened which receives the shadows. To the under part of 

 the box is fitted a ball and socket, by which it is attached to a stand which sup- 

 ports it; and the top or lid of it is fitted with hinges, in order that the box may 

 be laid quite open as often as it is necessary to alter any part of the machinery 

 it contains. The front of the box is also furnished with a falling lid or door, 

 moveable on hinges, by which the box is closed in front when it is not in actual 

 use. This instrument he calls a photometer. 



Finding it very inconvenient to compare 2 shadows projected by the same 

 cylinder, as these were either necessarily too far from each other to be compared 

 with certainty, or when they were nearer they were in part hid from the eye by 

 the cylinder: to remedy this inconvenience, he now made use of 2 cylinders; 

 which being fixed perpendicularly in the bottom of the box just described, in a 

 line parallel to the back part of it, distant from this back 2-^ inches, and from 

 each other 3 inches, measuring from the centres of the cylinders: when the 2 

 lights made use of in the experiment are properly placed, these 2 cylinders pro- 

 ject 4 shadows on the white paper on the inside of the back part of the box, 

 called the field of the instrument, 2 of which shadows are in contact precisely 

 in the middle of that field, and it is these 2 alone that are to be attended to. 

 To prevent the attention from being distracted by the presence of unnecessary 

 objects, the 2 outside shadows are made to disappear, which is done by rendering 

 the field of the instrument so narrow, that they fall without it on a blackened 

 surface, on which they are not visible. If the cylinders be each -^ of an inch 

 in diameter, and 2^ inches in height, as in the instrument he had lately con- 

 structed, it will be quite sufficient if the field be 2-;^ inches wide; and as an 

 unnecessary height of the field is not only useless, but disadvantageous, as a 

 large surface of white paper not covered by the shadows produces too strong a 

 glare of light, the field ought not to be more than -^ of an inch higher than 

 the tops of the cylinders. In order to be able to place the lights with facility 

 and precision, a fine black line is drawn through the middle of the field from the 

 top to the bottom of it, and another line horizontal or at right angles to it, at 

 the height of the top of the cylinders. When the tops of the shadows touch 

 this last-mentioned line, the lights are at a proper height; and when further the 

 2 shadows are in contact with each other in the middle of the field, the lights 

 are then in their proper directions. 



In his new-improved instrument (for he had caused 4 to be constructed) the 



