Management of Light in Illumination. 191 



that Important circumstance in the choice of the sub- 

 stances employed in constructing screens. 



In comparing two screens in order to discover which 

 of them is best calculated to answer the purposes for 

 which they are designed, they must be examined first 

 in respect to their powers of dispersing and softening 

 the direct rays of the flame of a lamp, and in the next 

 place in respect to the quantities of light which they 

 emit from their surfaces. 



It is not difficult to ascertain the first point with a 

 considerable degree of precision by simple inspection ; 

 but, where greater precision is required, the following 

 method may be employed : — 



Having placed before the photometer, at equal dis- 

 tances, two like lamps, burning with precisely the same 

 degree of intensity, and having masked them with the 

 two screens made of different substances which are to 

 be compared, a sheet of thick pasteboard is to be inter- 

 posed before each of these screens, and at the distance 

 of about one inch from it. This sheet of pasteboard 

 must be sufficiently large to mask the screen entirely 

 from the photometer, and it must have a circular hole 

 in its centre of about one inch in diameter, which must 

 be so placed that the centre of this aperture, the centre 

 of the flame of the lamp, and the middle of the field of 

 the photometer may be in the same right line. 



It is evident that in this situation of things little or 

 no light will arrive at . the field of the photometer but 

 that which comes from the flames of the lamps directly, 

 in straight lines, through the screens ; and by measur- 

 ing the relative intensities of those rays which arrive in 

 this manner through the two screens, by means of the 

 shadows and distances, it will be seen which of the 



