4 ecient The compensation is made by moving the mirror, 
AMERICAN . 
JOURNAL OF SCIENCE AND ARTS. 
[SECOND SERIES. ] 
Art. I.—Photometric Foaperiments. Part Srconp.—On the 
amount of light sraniinited by rein ae: ees crown glass 
a perpendicular incidence ; by 5, Po 
ee in Columbia Slee 
Tr would seem that inthe direct photometric experiments on 
this matter, the instrumental means employed have feare 
or less defective, in consequence of which the results al 
been determined, so that for perpendicular incidence, slender data 
exist for comparing the results of theory and cadets cata 
it will be s 
_ caused by two sources o light, T ieee deed a fethed in 
q reflected from a movable mirror, and pursues its way unob- 
structed to the screen, or is transmitted to it by the plate under. 
Am. Jour. Sct.—Szconp Serres, Vou. L, No. 148.—Juxy, 1870, 
: 1 
il 
