[ 475 ] 

 LXIV. Intelligence and Miscellaneous Articles. 



ON THE INTENSITY OF THE LIGHT REELECTED FROM GLASS. 

 BY DR. P. GLAN. 



TTITHERTO the investigations of the properties of reflected light 

 *--*- have been, by preference, occupied with the ratio of the two 

 principal components to each other, as well in respect of the phase as 

 of the intensity ; and only a few works have attempted a direct 

 determination of the two with respect to the incident light. The 

 only series of experiments on the intensity, which are found in 

 Arago's posthumous Works, for the white light reflected by a plane- 

 parallel glass plate, leaves out of consideration the absorption of 

 the glass ; and then it does not give the index of refraction or the 

 angle of polarization ; so that the results cannot well be compared 

 with the theory. On that account I have here undertaken again 

 this determination, in order before all things to test the correctness 

 of Cauchy's formula (coinciding with FresneFs) for light polarized 

 parallel to the plane of incidence. The observations could be limited 

 to the light polarized parallel to the incidence-plane, since from 

 numerous experiments the ratio between the two principal compo- 

 nents is known, and so with the determination of the first the 

 second is also given. 



The photometric method which I employed is based upon the 

 comparison of the brightness of two adjacent equally illuminated 

 fields, the intensity of which could be altered in a proportion known 

 from the construction of the aggregate. For this purpose a doubly 

 refracting prism was fixed to the collimator-lens of a theodolite so 

 that its principal section was parallel to the slit. The objective of 

 the observing-telescope carried a Mcol with a division-circle, which 

 gave readings to minutes; and the spectral analysis of the light 

 -was effected by the set of prisms of a Hofmann spectroscope fixed 

 upon the table of the apparatus. The slit was divided into two 

 parts by a strip of tinfoil, whose breadth was such that the ordinary 

 image of the one half and the extraordinary image of the other 

 were in exact contact ; and consequently two adjacent spectra were 

 obtained, the intensities of which, with equal brightness of the two 

 halves of the slit, were to each other as Tc sin 2 a to h cos 2 a, if a de- 

 notes the angle which the polarization-plane of the Mcol makes 

 with the principal section of the doubly refracting prism, and h and 

 Jc the coefficients of enfeeblement conditioned by the passage through 

 the apparatus of the two rays polarized perpendicular to one an- 

 other. 



In consequence of the dispersion of the doubly refracting prism, 

 perfect contact is only possible for one colour ; but the strong dis- 

 persion of the prisms of the Hofmann spectroscope makes that place 

 sufficiently wide for the maximum of sensitiveness to be obtained 

 for the colour in question. A slight inclination of the doublv 

 refracting prism about an axis perpendicular to its. principal section 



