785 



VOL. XC.] PHILOSOPHICAL TRANSACTIONS. 



farthest vane from the lamp, from double the logarithm of the distance of the 

 nearest vane. The remaining logarithm is that of the transmitting power, as 

 compared to the light coming directly to the eye from the other vane. I have now 

 only to remark, that the use of this instrument requires some practice, especially 

 when coloured glasses are to be examined ; it will however be found, that the dif- 

 ference of the colour of the 2 objects, when their light is brought to an equality, 

 may be overcome by a little abstraction, which is required for the purpose; for, 

 by attending only to brightness, it has often happened to me, that both objects 

 appeared at last of the same colour; which proved to be some mean between the 

 2 appearances considered separately. 



Some glasses stop so much light, that it will be adviseable to take them by the 

 assistance of an intermediate one. Thus, instead of comparing the open vane 

 directly to a red glass, I settle first the ratio of the violet one to that vane ; then, 

 taking the ratio of the red to the violet, and compounding these 2 ratios, the re- 

 sult will be more accurate. The reason for this will be easily comprehended, when 

 the construction of the apparatus is considered. For a red glass, immediately com- 

 pared to the open vane, would require its object to be brought extremely near the 

 lamp, while the other must remain at a very great distance. This would occasion 

 a considerable difference in the angles, both of incidence and of reflection, between 

 the rays falling on one vane, and on the other. But, by dividing the observation 

 into 2 operations, we avoid the errors that might be occasioned by the former 

 arrangement. In the following table, the 1st column contains the names of the 

 different substances through which light has been transmitted. The 2d column 

 shows the transmission of light, expressed in decimal fractions ; or the proportion 

 which it bears to the whole incident light considered as unity. An arithmetical 

 complement to this fraction, or what it wants to unity, will therefore give us the 

 proportion of light which is stopped by each of the substances contained in the 1st 

 column ; and that quantity multiplied by 1000 is placed in the 3d column. 



table 7* 

 Substances without colour. 



Transmission 



Bluish- white glass 914 



Flint glass 966 



Crown glass 797 



Coach glass 832 



Stoppage. Transmission. 



86 Iceland crystal 850 



34 Talc 910 



203 Easily calcinable talc 712 



168 



Very dark red glass 0001335 



Dark-red glass 000188 . . 999-8 



Orange glass 221 .. 779 



Yellow glass 681 .. 319 



Pale-green glass 465 . . 535 



Dark-green glass ....... .05 1 1 . . 949 



Bluish-green glass 231 . . 76'9 



Glasses of the prismatic colours. 



Transmission. Stoppage. 

 999-9 



Transmission. 



Pale-blue glass 3l6 



Dark-blue glass 199 



Indigo glass 000281 . 



Pale-indigo glass 0218 



Purple glass . 00675 



Violet glass 0452 



Stoppage. 

 150 

 90 

 288 



Stoppage. 

 . 684 

 . 801 

 i 9997 

 . 978 

 . 993 

 . 955 



VOL. XVIII. 



5H 



