O. N. Rood on Photometric Experiments. 5 
Differences from mean in per cents: 
No. 1 No. 2 No. 3. 4. No. 5 
+012 — 275 — 252 +573 — 102 
+'347 +126 — 137 — 208 +057 
+158 — 124 +°385 — 060 +041 
— 512 +275 +007 — 308 +005 
It will be observed that in the twenty experiments, only two 
eases occur in which the difference rises slightly above one half 
per cent, or above 3, of the whole quantity of light, while in 
nearly all the other cases the diferente falls considerably below 
; this ironies and the Byorage of their differences is less than 
_-0°2 per cent, or less than ;}5 of the whole amount. 
| As the reliability of measurements with a photometer is 
evidently connected with the power of the eye in distinguishing 
different degrees of brightness in adjacent surfaces, it is well to 
review here briefly the results that have been obtained by dif: 
ferent observers, as to the sensitiveness of the eye under favor- 
able conditions. 
The least difference which Boguer with his method, (two 
shadows.) was able to distinguish was ,); of the whole; ’ Fech- 
ner’s friends using the same method were able to reduce this 
quantity to ;};. Arago remarked that where one of the illumi- 
nated surfaces was in motion smaller differences could be per- 
ceived, and was able under favorable circumstances to dis-— 
tinguish ;1,;, while Masson, who used revolving discs, alo 
with his friends, was able to distinguish between ;'; to 74; 0 
the whole quantity of light. Near a window Helmholtz was 
‘able to distinguish with certainty a difference of 15, an 
sionally as small a quantity as ;34, while in the madi ofA a 
i Sieg p otie to sate ere = only ;},, seldom and 
t a good eye can distinguish a difference 
of ;; ona sevolving: dise, that the its eye would be able to 
obtain a similar result with ordinary forms of the photometer, 
where the conditions are far less favorable, and accordingly . 
seemed of interest to me to determine by a series of experiment 
the average sensitiveness of the eye while using the OA 
method described in this paper, particularly as the Pp an, of using 
* Physiologische Optik, von H. Helmholtz, page all. S55 
