6 O. N. Rood on Photometric Experiments. 
a single flame and mirror offered great facilities for making these 
observations in a reliable manner. ‘The following mode was 
employed: the light from the mirror was allowed to fall on the 
unobstructed screen, and the mirror was drawn up till the spot 
tered on separate fillets of paper a two pens. In the first experi- 
ment fourteen compensations were made by the advancing, and 
an equal number by the retreating mirror, and the mean of these 
twenty eight quantities was taken as the tru e distance of the 
r from the ‘screen ; arin ot com OP Pe the mean ses either 
stance, it was to as- 
by the eye under the given circumstances, In the first experi- 
ee this quantity was found to be ;+;, in a sec econd similar trial 
zi; of the total amount of light. The “screen ” in these experi- 
ments was quite | new, and an inferior result was obtained by 
using a “screen,’ ’ which had for six weeks been exposed quite 
unprotected to the action of the air; here the usual darkening 
of the edges had begun and progressed so far as to be faintly 
visible during compensation. 
In the experiments just detailed the highest nsec sensitive- 
ness of the eye is only ;3;, while as be fore show in pr ractical 
use the hes Ne difference from the mean is less ae x47 of the 
whole. ‘This higher degree of accuracy results naturally from 
the mode in sae the compensations are made, (alternately by 
Bt and recession,) and the fact that the compensations are 
us interwoven, in such a manner as to eliminate errors intro- 
duced by the slightly varying sensibility of the eye. 
Experiments on ae: oe of light transmitted a plates of glass 
t a perpendicular tnecidence. 
at knowledge of the amount of light transmitted and ag 
nd of in 
