1 76 Mr. Potter o)i a ne-jo Photometer by Comparison, atid 



arms round the pivot, to remain always perpendicular to the 

 plane of the board ; and hence, to an eye phiced as at d, these 

 glasses will reflect images of some portions of the surface of 

 the pasteboard. If this surface is everywhere equally illumi- 

 nated, the brightness of the reflection in the glasses will de- 

 pend only on their inclination to the visual rays : thus when 

 either glass reflects to the eye, the light from the part of the 

 pasteboard almost opposite, or from e, the reflection will be 

 very strong, and it will be weakest when the incidence is 

 nearly perpendicular, or when the glass shows to the eye the 

 part neary^ Having a fixed position for the eye, or a tube 

 through which to view the glasses, we easily determine the 

 angle at which the light entering the eye is incident on the 

 glass, by having a quadrant round the pivot graduated, and 

 showing the inclination of the arms to the direction of the 

 light. 



It will now be evident that to produce a representation of 

 the transmitted rings, we may view two narrow stripes of the 

 glasses, covering any superfluous parts with blackened paper, 

 and move the arms carrying them until the relative intensities 

 of the reflections are sensibly the same as the relative intensities 

 in the rings. Then knowing the angles of incidence upon the 

 glasses, we can calculate the intensities of the light from a for- 

 mula, which I have deduced from experiments, and published 

 in the Edinburgh Journal of Science. The apparatus pro- 

 ducing the rings with homogeneous light should also be at- 

 tached to the board, or otherwise kept so conveniently that 

 we may view them or their representatives alternately without 

 any considerable space of time intervening; and the paste- 

 board should be coloured to the same tint as the homogeneous 

 light made use of, to produce a more correct representation, 

 and to prevent the eye from being deceived by any difference 

 of colour. 



The lights I have used in these experiments are a good 

 homogeneous green, produced by a solution of arsenite of cop- 

 per (Scheele's green) in muriatic acid, and a very perfectly 

 homogeneous red, produced by a solution of iodine in hy- 

 driodic acid ; this last solution gives most probably a purer 

 colour than can be obtained of equal intensity by any other 

 medium. I keep the solutions in small cut-glass phials with 

 flat sides. 



With the green light I found the rings produced by a lens 

 of long focus* pressed upon a plane surface, to be represented 



* I do not know exactly the curvature of this lens, but believe it to be 

 to a radius of 15 or 16 feet. 



