on some Applications of' it to important OplicalPhdcnomena. 175 



duced by the interference of ^^^;th part of the whole vis viva, 

 of the transmitted light, must be allowed, on all mechanical 

 considerations, to fall exceedingly below so distinct an effect 

 as that which we witness. 



These considerations induced me to think upon the possibi- 

 lity of determining the relative intensities of the light in these 

 dark and bright rings. This is not so difficult a problem as 

 it at first sight appears to be ; for though we may perhaps 

 never expect to measure the light in the rings directly, yet it 

 will be seen that if we can form variable appearances where 

 we know the intensities of the lights, we may so vary them 

 that they may form a representation of the pha^nomena we are 

 considering; and then the only difficulty to be encountered is 

 that of the eye judging with sufficient precision and accuracy, 

 when the artificial is a correct representation of the natural 

 effect. In experiments of this sort the only resource is that 

 of repeated practice, by which the eye acquires a power of 

 judging with an exactness far beyond what would be expected 

 at the first trial. 



There are evidently many ways of producing appearances 

 where the intensities of the illuminations may be subjected to 

 measurement and calculation. The instrument 1 have ex- 

 ecuted for the particular purpose above mentioned, is very 

 manageable, but can only be used in the day-time, and in a 

 particular state of the sky ; that is, when it is either misty or 

 uniformly overcast with clouds, so that a uniform illumination 

 may be afforded to a piece of pasteboard, which is an essential 

 part of the instrument. 



The instrument I have used consists of a flat board of about 

 16 inches in length and 12 inches in breadth: on this board 

 I fix edgewise a rectangular 

 piece of pasteboard of about 

 23| inches in length and 3f 

 inches in breadth. The edge 

 of this pasteboard is fixed 

 along a semicircle described 

 on the board, as shown in the 

 figure. In the centre of the 

 circular arc is a pin, as at «, 

 upon which turn the two arms 

 a b and a c. Attached to each 

 of these arms is a piece of 

 crown-glass, which has been 



ground flat and polished; and afterwards covered at the 

 further surface with black varnish to prevent reflection there. 

 These pieces of glass are so fixed as when moving with the 



