Management of Light in Illumination. 181 



AS the art of illumination cannot be cultivated in a 

 satisfactory manner unless means are used for 

 measuring the light which is emitted by luminous 

 bodies, a photometer is indispensably necessary in 

 every experimental inquiry which is undertaken with 

 a view to the improvement of that art and of the vari- 

 ous instruments used in the practice of it. 



It is likewise necessary to adopt some fixed scale of 

 light to serve as a standard, which must be so arranged 

 as to indicate with certainty, by means of numbers, the 

 precise degree of illumination which takes place in any 

 given case, or the relative intensities of the lights which 

 are compared. 



This fixed scale of the photometer will be analogous 

 to the scale of the thermometer, but in one respect it 

 will be more perfect and more satisfactory: its inter- 

 vals, or degrees, may be made to measure very accu- 

 rately the different degrees of illumination they are 

 designed to indicate, whereas the degrees marked on 

 the scale of thermometers are arbitrary, and afford no 

 satisfactory information respecting the real difference 

 which exists in the various intensities of the heat 

 which they indicate. 



In my paper on the relative intensities of light 

 emitted by luminous bodies, which was read before 

 the Royal Society the 6th of February, 1 794, and 

 which is published in the Philosophical Transactions, 

 and also in the first volume of my Philosophical 

 Papers, an account is given of the photometer I used 

 in those researches ; but I have since found means 

 to simplify the construction of that instrument very 

 much, without injuring it in any respect, and have 

 added to it a graduated scale, which indicates the in- 



