O. N. Rood on Photometric Haperiments. 3 
Mode of adjusting the apparatus for experiment. 
1st. The center of the flame at L, and the center of the mir- 
ror M, must be at the same height above the common base. 
Qnd. The screen and plates to be examined are to be made 
perpendicular to the axis of the instrument, by the aid of the 
small gas flame mentioned in the first part of this ae the 
telescope T is also most readily collimated by the f the 
same flame, the screen, &c., having been temporarily nate. 
3rd. The mirror M- as ek must be brought into such a 
cpa that it shall send the ray MP perpendicularly to P, or 
along the axis of the instrument when its distance is such as to 
effect compensation ; for if the reflected ray were sent obliquely 
through re eround glass plate G, noticeable errors woul 
introduced. This adjustment, which is important if good results 
are expected, is best effected by making one or two compensa- 
tions so as to determine approximately the correct distance of 
the mirror, which is then rotated on its vertical axis so that the 
reflected image of the spears is made to fall in the center of the 
field of the telescope, the screen having been previa pore 
a ee When, ho pele pad os 
ucin 
pensation cn int itself has Bes ‘shifted by the introduction ol 
removal .of a plate of glass, the mirror will corresponding] 
moved away or toward 8, and of course the ray MP ey be 
sent a little to the right or left, and it becomes necessary to de- — 
vise some simple way in which this difficulty can be o vate, a 
without in each mites removing the sereen and altering the 
focus of the telesco : . 
The mirror then ae actually in adjustment, , this, when lost, 
can be recovered as follies ws: in the screen B about four inches 
in 
throu. a it with a ue eye, and the image of the flame is 
seen er up on the mirror, and is made to coincide with a 
- short black line pet there previously. The ray MP can al- 
ways thus be made to coincide with the axis of the inagronent a 
it not even being necessary for the observer to leave his seat or 
ish the sensitiveness of the eye by exposure. I am par- 
ticular in dese this precaution, having in my own case re- 
jected the results of 1800 compensations which were made with 
a comparative lack of attention to this single point. 
a 
