62 O. N. Rood on a Photometer. 
in such a way that the surface of the upright is reduced appar- 
ently to a line, when the lamp is moved on the slide till this line 
bisects the flame. ‘The slide is then placed about as distant from 
the screen as may be desired and, compensation approximately 
effected, by moving the pair of lamps at L away from or towards 
the collodion surface. If these are brought too near the screen, 
the square spot will appear dark on a light ground; if too far 
off, light on a dark ground. The final compensation is made 
by moving the slide lamp S$; this takes place when the spot 
omes invisible. In the first experiments, instead of a pair, a 
single lamp was used, and owing to this the spot never became 
entirely invisible; one half was always faintly lighter than the 
adjacent ground, the other half darker; this was owing to the 
slight difference in the distances of the sides of the square from 
the lamp, and may be adduced as a proof of the delicacy of the 
photometer. The observer then reads off on the scale the dis- 
tance of the slide lamp from the ground glass surface next to 
the collodion plate, the white page of the note book reflecting 
sufficient light for the purpose. Supposing it is desired, for ee 4 
ample, to measure the amount of light transmitted by a plate 
polished glass at a perpendicular incidence, the plate is placed 
at P, when it will be found necessary to bring the slide lamp 
nearer in order to effect compensation. The amount of light — 
transmitted is then at once calculated by the law that “the in- 
tensities which measure the values of the illuminating powers 
are directly as the squares of the distances from the screen. 
My mode of work, in all cases, has been to alternate the obser- 
vations, reading off first the distance of the free flame; 2d, that 
7 sil colored light are on the whole 
by no means reach the accuracy easily attained when the tw’ 
_ masses of light are similar in tint. ith regard to the follow 
ing observations, it may be remarked, that those made with col 
retty satisfactory, still they 
