906 PEOFESSOE BTJNS®^ AJSTD DE. H. E. EOSCOE’S PHOTOCHEMICAL EESEAECHES. 
exactly in the middle of the sun’s image. The fine hole in the plate must be very care- 
fully bored, as its area has to be accurately determined. ^ e found that the best way 
of obtaining a perfectly circular opening was to lay a thin piece of copper foil upon a 
thick sheet of tin or lead, and to bore a small hole through the copper with a fine 
needle, afterwards rubbing the foil upon a moistened hone to separate all irregularities, 
and boring carefully with the needle, until, under the microscope, the edge of the hole 
was seen to be sharp and circular. 
The diameter of the fine opening was determined by measui'ement with a micrometer, 
and the distance from the insolation-vessel to the point on the shutter of our dark room, 
where the opening was placed, was also determined. K we suppose that this distance 
be called 5, and the diameter of the opening the apparent diameter (a) of the opening, 
as seen from the insolation-vessel, is fomid from the formula 
, d 
tan — 2^' 
The square of this apparent diameter (A) is to the square of the apparent diameter of 
the sun (Xj), as the observed action is to the action which the sun would have produced 
if it had shone direct upon the insolation-vessel. In order to obtain the total action 
proceeding from the sun, it is necessary to multiply the observed action by the factor 
The factor for the transformation of the direct observations into units of light is, as in 
the former calculations, 
%= 2 - 249 . 
The factor N, by help of which the experimental results are made independent of 
refiexion in the insolation-vessel, is found from the formula (I.), 
when and 10““*= 7, and when ^ signifies the coefficient of reflexion from glass 
into air, h the thickness of exposed sensitive gas in the insolation-vessel, and a the coeffi- 
cient of extinction of direct sunlight for the chlorine and hydrogen mixture. As § and li 
were known from our former experiments, we had only to determine the value of a ; tliis 
was accomplished in the following way. Between the small opening in the shutter and 
the insolation-vessel, we placed one of our transparent cylinders * filled with air, and then 
measured the amount of action (Wi) effected by the direct sunlight falling on the instru- 
ment. The empty cylinder was next replaced by a similar one filled, the usual precau- 
tions being adopted, with the mixture of chlorine and hydrogen, and the action (w) again 
determined. After removal of this cylinder, the former one containing air was replaced 
and the action once more measured, and. the mean of the three times of observation 
noted. From the first and third observations, with the cylinder containing air, the 
* Philosophical Transactions, 1856, p. 357. 
