Luminous Intensity of Light. 



369 



light, the polarized disks d\ d would be reddish and the disks c, c' greenish, 

 the central disk c d being of the tint formed by the union of the two 

 shades. The analyzing prism K, and the selenite disk I, will detect free 

 polarization in the disk c d, if it be coloured, as readily as if it were white ; 

 the only difference being that the two disks of light, g, r, cannot be brought 

 to a uniform white colour when the lights from D and C are equal in 

 intensity, but will assume a tint similar to that of c d. When the con- 

 trasts of colour between D and C are very strong — when, for instance, one 

 is bright green and the other scarlet — there is some 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. 



No attempt has been made in these experiments to ascertain the exact 

 value of the standard spirit-flame in terms of the Parliamentary sperm- 

 candle. Difficulty was experienced in getting two lots of candles yielding 

 light of equal intensities ; and when their flames were compared between 

 themselves and with the spirit-flame, variations of as much as 10 per cent, 

 were sometimes observed in the light they gave. Two standard spirit- 

 flames, on the other hand, seldom showed a variation of 1 per cent., and 

 had they been more carefully made they would not have varied O'l per 

 cent. 



This plan of photometry is capable of far more accuracy than the pre- 

 sent instrument will give. It can scarcely be expected that the first 

 instrument of the kind, made by an amateur workman, should possess 

 equal sensitiveness with one in which all the parts have been skilfully made 

 with special adaptation to the end in view. 



Addendum to description of Photometer. By W. Crookes, F.R.S. 

 Received December 17, 1868. 



When I wrote that other experimentalists had already made use of the 

 phenomena of polarized light for measuring the intensity of light, I was 

 not aware that a photometer already existed in which the principle of the 

 one above described was adopted. 



By the kindness of Sir Charles Wheatstone I have, within the last few 

 days, been enabled to experiment with a photometer devised by M. Jamin, ' 

 founded on the same principle. I have not yet succeeded in finding a 

 printed account of this instrument, but a written one was supplied with it, 

 and having been allowed to take it to pieces its construction is evident. 



It consists, first, of a Nicol's prism, then of an achromatized doubly 

 refracting prism ; next, of two plates of quartz, cut oblique to the axis, 

 reversed, and superposed ; and finally, at the eye-end, of a second Nicol's 

 prism. As in my instrument, each of the two lights to be compared split 



