ar Jor the Advancement of Srience ee 401 hae 
Prof. Stokes observed. that’ heretofore it had Lestiinlagrllyaiat 
posed that the clouds always exerted a aegolenning: action on the Tight 
On the Danselite by) a ‘nantes 
4 Be 
portance of distin siahinig n the optical. foci of the lenses used 
in photographic cameras and the foci of the photogenic rays.» He 
said that ignorance of this distinction, or inattention to it, the 
had invented a simple instrument, which was exhibited and: exple 
ed, for accurately distancing the object to be depicted and: determi 
the corresponding foci of the photogenic rays in any given camera. 
s 
proper 
for exposing an sph on a given day, aint un 3 r given ae 
are camera; and that a longer or shorter time than this was 
injurious to the effect... To ascertain readily this proper cal he had 
Invented dynactinometer, which he now exhibited. . It consisted of 
a square frame of card, Bho a circle of card capable of being turned 
round either by hand or by clock-work ; in one position of this circle, the, . 
whole surface of the rit exposed to the camera at the proper p 
genic distance was black.;. but as the circle turned, a. neatly divided“: 
Sector of white card was eX] d, and by causing the circle to turn so 
as to expose a given number of i tins each successive equal num- 
t of seconds, the part of the sector whose image was most clearly 
_ @efined on examination of the photogenic ee gave the number of 
~ Seconds best for exposing the object to the camera. © But as the several 
photogenic plates were not equally sensitive, thie sensitiveness of the plates 
was determined by placing them in a small frame, and allowing them to 
descend along an inclined plane, during a Simp de of which descent 
i i rest being 
ete 
melee. 
ct means of comparing the sensitiveness of the several plates. 
Attempt to eicsate the occasional distinct vision if ie revolving 
ored sectors; by Prof. Steve 
He exhibited an instrument for whirling cards with coletedaes sectors 
on them, devise d by Mr. ee of Belfast, to teach his children jetta 
ads 
Proportion to apes e the exact bron whieh they Sociol This 
apparatus he had lent to Prof. Stevelly to show his class; and while 
doin, ng so, he was surprised to observe that while the cards were revolvin 
Tapidly, if he suddenly turned away his head he caught a distinct view 
of the individual colored sectors at the instant he was losing sight of 
‘Srconp Serres, Vol. X, No. 30.—Nov., 1850. 51 
which they reflected or transmitted to the eye. eight ss a 
