IQO Management of Light in Illumination. 



In order to determine with the greatest precision the 

 quantity of light which is lost in passing through a 

 screen, two Argand lamps, placed at equal distances 

 before the photometer, and having been made to burn 

 with precisely the same degree of intensity, the shadows 

 projected in the field of the instrument will be of the 

 same density. If now a screen be interposed before 

 one of these lamps, the shadow belonging to it will 

 become a little less dark than the other shadow. On 

 moving the lamp, which is covered by a screen, a little 

 nearer to the photometer, the equality of density of the 

 shadows will be restored ; and, when that has been done, 

 the divisions of the scale of the photometer will indi- 

 cate the intensities of the light, and the difference of 

 the intensities indicated will show the quantity of light 

 destroyed in passing through the screen. 



As the object principally had in view in using a 

 screen is to disperse the direct rays of a too powerful 

 flame, it is evident that the less the flame is seen 

 through the screen (the total quantity of light diffused 

 remaining the same), the better it performs its office ; 

 but, as the flame is always seen more or less distinctly 

 through a screen, it is certain that a considerable 

 portion of the light diffused does not come from 

 the screen, but directly through it from the flame in 

 straight lines. Now as it is very certain that two 

 screens of the same form and dimensions, but com- 

 posed of different substances, may moderate the inten- 

 sity or brilliancy of the direct rays from a powerful 

 flame in the same degree, and yet the total quantities 

 of light sent off from the surfaces of these screens by 

 which surrounding objects are illuminated may be very 

 different, it is necessary to pay particular attention to 



