1868.] 



of the Luminous Intensity of Light* 



167 



The instrument devised for measuring the relative intensities of the 

 standard and other lights is next described ; it has this in common with 

 that of Arago described in 1833, as well as with those described in 1853 

 by Bernard, and in 1854 by Babinet, that the phenomena of polarized 

 light are used for effecting the desired end*. But it is believed that the 

 present arrangement is quite new, and it certainly appears to answer the 

 purpose in a way which leaves little to be desired. The instrument cannot 

 be described without the aid of drawings which accompany the original 

 paper, but its mode of action may be understood by the following descrip- 

 tion. 



The standard lamp being placed on one of the supporting pillars which 

 slide along a graduated stem, it is moved along the bar to a convenient 

 distance, depending on the intensity of the light to be measured. The 

 light to be compared is then fixed in a similar way on the other side 

 of the instrument. On looking through the eyepiece two brightly lumi- 

 nous disks will be seen, of different colours. One of the lights must now be 

 slid along the scale until the two disks of light, as seen in the eyepiece, are 

 equal in tint. Equality of illumination is easily obtained ; for, as the eye 

 is observing two adjacent disks of light which pass rapidly from red-green 

 to green-red, through a neutral point of no colour, there is no difficulty in 

 hitting this point with great precision. Squaring the distance between 

 the flames and the centre will give inversely their relative intensities. 



The delicacy of this instrument is very great. "With two lamps, each 

 about 24 inches from the centre, it is easy to distinguish a movement of 

 one of them to the extent of one-tenth of an inch to or fro,and by using 

 the polarimeter an accuracy exceeding this can be attained. 



The employment of a photometer of this kind enables us to compare 

 lights of different colours with one another. So long as the observer, by 

 the eyepiece alone, has to compare the relative intensities of two surfaces 

 respectively illuminated by the lights under trial, it is evident that, unless 

 they are of the same tint, it is impossible to obtain that absolute equality 

 of illumination in the instrument which is requisite for a comparison. By 

 the unaided eye one cannot tell which is the brighter half of a paper disk 

 illuminated on one side with a reddish, and on the other with a yellowish 

 light ; but by using the photometer here described the problem becomes 

 practicable. When the contrasts of colour are very strong (when, for 

 instance, one is a bright green and the other scarlet) there is difficulty 

 in estimating the exact point of neutrality ; but this only diminishes the 

 accuracy of the comparison, and does not render it impossible, as it would 

 be according to other systems. 



* Since writing the above, I have ascertained that M. J amin had previously devised 

 a photometer in which the principle adopted in the one here described is employed, 

 although it is carried out in a different and, as I believe, a less perfect manner. — • 

 W. C, Dec. 16, 1868. 



