Management of Light in Illumination. 183 



cylindrical columns, made of wood, each | an inch in 

 diameter and 2 inches in height. 



The centres of the holes made in the table for re- 

 ceiving these columns are at the distance one inch and 

 three quarters, the one on the right hand and the other 

 on the left, from the horizontal line which crosses the 

 table from the front to the back part of it. Con- 

 sequently the cylinders are at the distance of three 

 inches from each other, and the centre of each of them 

 is three inches from the vertical line which is drawn in 

 the middle of the field of the photometer. 



The whole of this simple apparatus may be con- 

 structed of beech-wood ; and it may be stained of a 

 fine deep black colour by washing it several times 

 with common writing ink. It must be made quite 

 black, and it will be better to stain it than to paint it 

 with oil colours. 



The scale of this instrument is composed of long 

 rulers, each one inch wide and above a quarter of an 

 inch thick, with a circular hole of about half an inch in 

 diameter within about half an inch of one of its ends. 

 This hole is made to receive one of the cylindrical 

 columns of the photometer, by means of which it is 

 confined in its place when in use. These rulers serve 

 to measure the distances of the lights which are the 

 subject of an experiment, from the centre of the field of 

 the photometer. 



A few words will be sufficient to give such clear and 

 distinct ideas of the nature of these experiments, and of 

 the manner of performing the various operations they 

 require, as will enable any intelligent person not only 

 to construct the necessary apparatus, but also to use it 

 with the greatest facility and success. 



