measuring the comparative Intensities of Light. 81 
by removing entirely the black pasteboard screen from before 
this plane, or making use of another with a larger aperture ; 
which is sometimes advantageous.* 
Having now, as I imagine, sufficiently described all the es- 
sential parts of these instruments, it remains for me to give 
some account of the precautions which, from experience, I 
have found it necessary to take in making use of them. 
And first, with respect to the distance at w^iich lights whose 
intensities are to be compared should be placed from the field 
of the photometer, I have found that when the weakest of the 
lights in question is about as strong as a common wax-candle, 
that light may most advantageously be placed from 30 to 36 
inches from the centre of the field ; and when it is weaker or 
stronger, proportionally nearer or farther off. When the lights 
are too near, the shadows will not be well defined ; and when 
they are too far off, they will be too weak. 
It will greatly facilitate the calculations necessary in drawing 
conclusions from experiments of this kind, if some steady light 
of a proper degree of strength for that purpose be assumed as 
a standard, by which all others may be compared. I have 
chosen for that purpose an Argand’s lamp, made in London, 
and very well finished ; and though the quantity of light 
emitted by this, or any other kind of lamp, is very various, 
* Since writing the above, I have made a little alteration in the form of the box 
which contains my photometer. The front of it, instead of being open, is now closed, 
and the light is admitted through two horizontal tubes, which are placed so as to form 
an angle of 6o° ; their axes meeting at the centre of the field of the instrument. The 
held of the photometer is viewed through an opening made for that purpose in the 
middle of the front of the box, between the two tubes abovementioned. The annexed 
figures, lab. X. XI. XII. XIII. will serve to give a clearer idea of the instrument, as 
it now is, in its most improved state. 
MDCCXCIV. M 
