ss) 
soft 
ne 
B. Silliman, Jr. and C. H. Porter ona New Photometer. 315 
Arr. XXXII.— Notice of a Photometer and of some experiments 
therewith upon the comparative power of several a ee eee 
onl Llumination ; by B. Sinumy, Jr., and Cuas. H " Poieion, 
Rye E photometers in general use for determining the compara- 
tive illuminating power of various sources of light, depend upon 
a comparison either of shadows or of illuminated discs. Rum- B. 
ford’s well known instrument is of the former pete ca a and ag 
Ritchie’s is an example of the latter. Having found much diffi 
etl in the adaptation of any of the instruments ¢ vena pA in 
_ Use to the accurate admeasurement of the peob Ts power of 
_ various forms of gas jets, whee B. Silliman, Jr. was led to con- 
Messrs, J. & New Haven, opticians of emi- 
them to give form to our 
mutual conclusions by con- 
Rok nla the instrument here 
 teecribed, 
Poe ption of the instrument. 
ata this i instrument the pen- 
cils of light are received 
prisms of flint glass on 
he angle of total reflection. 
+. ved 
 ‘“pona diaphragm of ground 
Be se peti ina dark cham- 
m.m. in diameter. 
- Figure 2 shows ne diaphragm — 
i plan with the two dises dd’ whe 
i. Telative position. Above - 
8 is the eye-piece o 
wit 
instra ument is [Seale 4] 
ae aes 
