360 On the Measurement of the [July, 
arrived at the general principle upon which my photometric method 
is entirely founded. The quantity (I do not say the proportion)— 
the quantity of completely polarized light, which forms part of a beam 
partially polarized by reflection, and the quantity of light polarized 
rectangularly which is contained in the beam transmitted under 
the same angle, are exactly equal to each other. The reflected 
beam and the beam transmitted under the same angle by a sheet 
of parallel glass, have in general very dissimilar intensities; if 
however we examine with a doubly refracting crystal, first the re- 
flected and then the transmitted beam, the greatest difference of 
intensity between the ordinary and the extraordinary images will 
be the same in the two cases, because this difference is precisely 
equal to the quantity of polarized light which is mixed with the 
common light.” 
In Arago’s astronomy, the author again describes his photo- 
Fic. 1 meter in the following words: “I have con- 
oN) structed an apparatus by means of which, upon 
operating with the polarized image of a star, 
we can succeed in attenuating its intensity by 
D 0 degrees exactly calculable after a law which I 
have demonstrated.” It is difficult to obtain 
an exact idea of this instrument from the de- 
— scription given ; but from the drawings it would 
See appear to be exceedingly comaehiied and to 
be different in principle and construction from 
the one now about to be described. The pre- 
ee’ — sent photometer has this in common with that 
of Arago as well as with those described in 
1853 by Bernard,* and in 1854 by Babinet,f 
gd that the phenomena of polarized lght are 
used for effecting the desired end. But it is 
—_=—==»——-1 believed that the present arrangement is quite 
new, and it certainly appears to answer the 
purpose in a way which leaves little to be de- 
sired. The instrument will be better under- 
x stood if the principles on which it is based are 
first described. 
Fig. 1 shows a plan of the arrangement of 
parts, not drawn to scale, and only to be re- 
garded as an outline sketch to assist in the 
comprehension of general principles. ‘Let D 
represent a source of light. This may be a white disc of porcelain 
or paper illuminated by any artificial or natural light. C represents 
* “Comptes Rendus.’ April 25, 1853. 
+ ‘ Proceedings of the British Association,’ Liverpool Meeting, 1854. 
