Luminous Intensity of Light. 



365 



must be arrested at the moment when, 'm this growing extraordinary 

 image, the segment corresponding to the part of the moon illuminated by 

 the sun exhibits the intensity of the ashy part shown by the ordinary 

 image. From these data it is easy to perceive, he says, that the problem 

 is capable of solution. 



In another part of the same volume, after speaking of the polariscope 

 which goes by his name, Arago writes: — "I have now 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, how- 

 ever, we examine with a doubly refracting crystal first the reflected 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 photometer in the 

 following words : — " I have constructed an apparatus by means of which, 

 upon operating with the polarized image of a star, we can succeed in at- 

 tenuating its intensity by degrees exactly calculable after a law which I 

 have demonstrated." It is difficult to obtain an exact idea of this instru- 

 ment from the description given ; but from the drawings it would appear 

 to be exceedingly complicated and to be different in 

 principle and construction from the one now about to 1 * 

 be described. The present photometer has this in com- GDC*) 

 mon with that of Arago, as well as with those de- b c 



scribed in 1853 by Bernard *, and in 1854 by Babinet f, 

 that the phenomena of polarized light are used for effect- h£^3 

 ing 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 will be better understood if \ZJ viAjL/ 

 the principles on which it is based are first described. 



Fig. 1 shows a plan of the arrangement of parts, not i — t — 

 drawn to scale, and only to be regarded as an outline 

 sketch to assist in the comprehension of general prin- 

 ciples. Let D represent a source of light. This may t^^3 K 

 be a white disk of porcelain or paper illuminated by 

 any artificial or natural light. C represents a similar (J) (j^) 



white disk, likewise illuminated. It is required to com- 

 * Comptes Kendus, April 25, 1853. 



f Proceedings of the British Association, Liverpool Meeting, 1854. 



