﻿228 Royal Society .— 



of sp. gr. 0*805, and pure benzol boiling at 81° C, which are mixed 

 together in the proportion of five volumes of the former and one of 

 the latter. The wick-holder consists of a platinum tube, and the 

 wick is made of fifty-two pieces of platinum wire, each 0*01 inch in 

 diameter. The flame of this lamp forms a perfectly shaped cone, 

 the extremity being sharp, and having no tendency to smoke ; with- 

 out flicker or movements of any kind, it burns when protected from 

 currents of air at a uniform rate of 136 grains per hour. 



There is no doubt that this flame is very much more uniform than 

 that of the sperm-candle sold for photometric purposes. Tested 

 against a candle, considerable variations in relative illuminating 

 power have been observed ; but on placing two of these lamps in op- 

 position, no such variations have been detected. 



The instrument devised for measuring the relative intensities of 

 the standard and other lights is next described ; it has this in com- 

 mon 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 description. 



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 eye- 

 piece two brightly luminous 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 illu- 

 mination 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 com- 

 pare lights of different colours with one another. So long as the 

 observer, by the eyepiece alone, has to compare the relative inten- 

 sities of two surfaces respectively illuminated by the lights under 

 trial, it is evident that, unless they are of the same tint, it is impos- 



* Since writing the above, I have ascertained that M. Jamin 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. 0., Dec. 16, 1868. 



