y\NT) TRANSMITTED BY GERl’ATN KINDS OE GLASS. 
247 
Experiments were also made to ascertain whether repolishing altered in any way 
the reflective power of the glass ; and the polarising angles before, and after, repolisli- 
ing were also determined. 
The observations were made with five plates of Messrs. Chance’s “lighthouse” 
glass, for which the author is indebted to Mr. Kenwahd, the plates being G‘5 mm., 
ir5 mm., 15 mm., 18'5 mm., and 24’3 mm. thick respectively, and with different 
thicknesses of Messrs. Field’s ordinary dense flint.” A block of this glass was 
procured from Mr. Hilger, by whom a slice 7 mm. thick was cut off and polished, 
and then the remainder of the piece formed into a rectangular block measuring 
9r3 mm. X 69'5 mm. X 49 mm., and the six faces carefully polished. 
Some measurements were also made with a piece of ordinary plate glass 6 mm. 
thick. 
The ordinary plate glass v^as green when seen edgewise, Messrs. C^hance’s glass 
was slightly green when viewed in the same manner, and the flint glass was distinctly 
yellow by daylight. 
Mr. Ken ward states that “ the lighthouse glass is of a special mixture, varying 
slightly from time to time; it is of the nature of hard crown glass.” The average 
refractive index for the sodium line of Messrs. Chance’s hard crown is stated by them 
to be 1‘5172, and of their soft crown 1’5146 ; and in a letter which accomjianied the 
glass its index was said to be “about 1’51 or 1'52.” 
The refractive indices for the sodium line of the crown glass, the flint glass, and 
the plate glass used in these experiments were determined in the ordinary way with 
small prisms made of each kind of glass by Mr. Hilger ; the values found were :— 
Crown glass. 1'5145, 
Flint glass. 1'6330, 
Plate glass. 1'5274. 
The indices of the crown and plate glass were also determined witli the wedges of 
these two kinds of glass used for the reflection experiments; that for the crown was 
found to be 1‘5137, and of the flint 1'6385 ; the refracting angles of these two wedges 
heing only 9° 39' 45” and 9° 51' 19”, whilst those of the small prisms were 59° 45' 19” 
and 59° 45' 17”, it seemed probable that the values obtained with the latter were 
the most accurate, and they were therefore taken as the true values of the indices. 
The account of the exj)eriments is given in Part I., and the results deduced from 
them in Part II., the account of the experiments being divided into six sections :— 
1. Amount of light transmitted. 2. Amount of light reflected at a nearly perpen¬ 
dicular incidence. 3. Amount of light reflected at a nearly perpendicular incidence 
after repolishing. 4. Amount of light reflected at various incidences between 0° and 
90° by the crown glass before and after repolishing. 5. Amount of light transmitted 
after repolishing. 6. Values of the polarising angles before and after repolishing. 
