■33 



MA TURE 



[ June 5, 1884 



On Silvered Glass Mirrors. — The first set of photometric de- 

 terminations were made with a silver film deposited <>n a flat 

 and well-polished glass plate 76-5 mm. long and 51 mm. wide. 



The glass plate was weighed before and after being coated 

 with silver, and the weight of the film was found to be 

 0-0035 grin. ; assuming the density of the silver to be io'62, 

 that being the value for silver finely divided by precipitation 

 given in "Watts Dictionary," vol. v. p. 277, the thick- 

 ness of the film calculated from the area and weight was 

 0-00008447 mm. 



'the film appeared opaque by ordinary daylight, but when 

 examined with sunlight was seen to be slightly transparent and 

 of a deep blue colour. 



The photometrical determinations were made in exactly 

 the same way as those with the speculum metal and itei 

 mirrors (Free. Roy. See. vol. xxxvi. p. 187), and the observa- 

 tions were about as concordant as those contained in the Tables 

 I. and II. of the paper giving an account of the experiments. 



Two complete series of observations were made with light 

 polarised in, and perpendicularly to, the plane of incidence, 

 the results are given in Tables I. and II. 



The angles of incidence are given in the first column, the per- 

 centage amount of light reflected in the second and third, the 

 means of the two sets of observations in the fourth, and the 

 amount of light which ought to have been reflected according 

 to Cauchy's formula? in the fifth. 



TABLE I.— Silver Film, with Light Polarised in the Plane of 

 Incid 



Table II. — Silver Film, with Light Polarised Perpendicularly 



to the /Vane of Incidence 



For light polarised perpendicularly to the plane there is con- 

 siderable difference between the two sets of numbers, the calcu- 

 lated values being considerably the highest. 



As has already been stated, the silver film was, to some extent 

 at least, transparent, and it was found that when a Nicol was 

 held between the eye and the silvered glass, and sunlight was 

 incident obliquely upon the film, the brightness and colour of 

 the transmitted light varied with the position of the Nicol ; the 

 image of the sun being brightest when the short diagonal of the 

 Nicol was in the plane of incidence, and darkest and of a deep 

 blue colour when the long diagonal was in that plane. Hence 

 it would appear that at oblique incidences light which is polar- 

 ised perpendicularly to the plane of incidence penetrates to a 

 greater depth in the film than that polarised in the plane — a 

 result that is in accordance with the conclusion drawn from the 

 experiments with silver films already referred to, and one that 

 may account for the difference in the observed and calculated 

 intensities of light polarised perpendicularly to the plane ol 

 incidence reflected by the silver film. 



In order to ascertain whether the difference between the ob- 

 served and calculated results was really due to this cause or not, 

 a thicker film was prepared by depositing a second coating of 

 silver on a freshly-prepared film. 



The same glass plate was used ; the silver weighed 0-0072 

 grin., and its thickness was therefore 0*0001737 mm., or as 

 nearly as possible double that of the single film. 



The thick film was not absolutely opaque, as the disk of the 

 sun on a clear day could just be seen through it, but it trans- 

 mitted much less light than the film previously used. 



Tallies IV. and V. give the results of two series of observa- 

 tions made with it, and also the theoretical amount of light 

 which should have been reflected, calculated from the values 

 of the principal incidence and principal azimuth given in 

 Table VI. 



Table IV. — Double Silver Film, with Light Polarised in the 

 Plane of Incidence 



The principal incidences and the principal azimuths were de- 

 termined, an 1 the in. ans of two sets of eight observations each 

 en in Table III. 



The values of the principal azimuths are higher than any ob- 

 taiin 1 before in the course of these experiments, whilst those of 

 the principal incidences are nearly the same as those obtained 

 with the silver plate polished with rouge (Proc. Roy. Soc. vol. 

 xxxi. p. 493), but considerably in excess of the determinations 

 previously made with silver films. 



Table III. 



Pi incipal incidence Principal azimuth 



75 38 44 07 



75 36 43 40 



Mean ■• 75 37 43 53 



The calculated and observed values for the light polarised in 

 the plane of incidence agree very fairly, the calculated values 

 being slightly the lowest. 



Table V. — Double Silver Film, with Light Polarised Perpen- 

 dicularly to the Plane of Incidence 



75 5o 

 75 45 



43 52 



44 07 



Mean ... 75 47 ... ... ... 44 o 



The values of the principal incidence and azimuth are slightly 

 higher than those obtained with the thinner film, and therefore 

 the percentage amount of light which, according to theory, 

 should be reflected by the silver, is also higher. 



