 % O. N. Rood on Photometric Eaperiments. 
Mode of measuring the amount of light transmitted by a plate of 
glass, ce. 
1st. The Shear is placed at P and collimated by the use of the 
small gas flam 
2nd. A Pptapanmetion is effected by moving the mirror 
8rd. The mirror is adjusted by the use of the aperture in B, 
and another ee made, followed by a second adjust- 
ment of the mirror, if n 
4th. A series of Seseful See are now made, alter- 
nately by the het oF and recession of the mirror ; these are regis- 
mirror by the aid of the aperture in ‘die: screen B. Finally, a 
series of compensations are effected with the free flame and 
registered. 
At this point it may be well to taclicean an objection which 
might be urged to this mode of experi , Viz: itis evident 
that the angles at which the light is Pretegted from the mirror 
not be identical in the presence and absence of the plate of 
glass, and as the amount of light reflected by glass and silver 
varies with the angle of reflection, this might become a source 
less glass employed by me, however, the angle in one case was 
at B07 an nd in the other 5° 18’, the difference of 12’ being much 
roduce an appreciable effect, as we know from the 
eats of ot oes experiments 
For the purpose of showing what can be effected by this 
method, I can perhaps do no better than give the results of five 
sets of experiments; each of them consisted of four sets of eight 
double, or sixteen single compensations, and during the progress 
of each trial the conditions remained the same. The ex peri- 
ments were made as follows: eight double compensations were 
effected with the a and registered ; then nboabdcres: 
ing arranged as before, a new set of compensations were 
— ke, all of which was repeated four times. The Rican? 
mean of the distances of the source of light from the a: was 
then obtained separately for each of the four experiments, and 
from these four mean values of the distance, a final mean aie 
was deduced. This last quantity was then squared and com- 
ly with the squares of each of the first four mean 
ing tells at once the difference of the 
; mean result, in per cents or fractions 
te eapied to Elves an idea of the degree of ac- 
sted in this kind of observation. 
