8 Experiments on the Relative Intensities 



ows projected by the same cylinder, as these were 

 either 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 incon- 

 venience I now make use of two 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 two lights made use of in the experiment are 

 properly placed, these two cylinders project four shad- 

 ows upon the white paper upon the inside of the back 

 part of the box, which I shall henceforth call the field 

 of the instrument, two of which shadows are in contact 

 precisely in the middle of that field; and it is these 

 two alone that are to be attended to. To prevent 

 the attention being distracted by the presence of un- 

 necessary objects, the two outside shadows are made 

 to disappear, which is done by rendering the field of 

 the instrument so narrow that they fall without it, 

 upon a blackened surface, upon which they are not 

 visible. 



If the cylinders be each -fa of an inch in diameter, 

 and 2 T 3 (7 inches in height (as they are in the instru- 

 ment I have lately constructed), it will be quite suffi- 

 cient if the field be 2^-5 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 



